So apparently I heard from here, here and here about the differences in PhDs in the US and in Europe.
To sum up, it takes longer, on average, in the US than in Europe to finish a PhD since US PhD programs require less and have more coursework compared to Eur PhD programs.
Why is that so?
I tried looking it up but seemed to be getting the stuff above, nothing really explaining why that is so.
First, let me start off with a counter-question:
Why would you expect them to be exactly the same?
Different regions have historically different educational systems on all levels, starting from Kindergarten. Why would you expect specifically the PhD degree to have a completely uniform definition everywhere in the world? Of course there are nowadays activities to make degrees (incl. PhDs) more comparable worldwide, but these things take time. As politicians in Europe have learned as part of the so-called Bologna process, you can't just top-down decide that from now on, we are using the US system.
Of course, there are sometimes reasonable arguments for differences in system. For instance, you concluded:
it takes longer, on average, in the US than in Europe to finish a PhD since US PhD programs require less and have more coursework compared to Eur PhD programs.
In the US, a bachelor's degree is required for starting a PhD. In Europe, almost universally, you need a master or one of the older five-year diploma studies. So we in Europe expect students to hit the ground running basically from day 1 in their PhD. On the other hand, we don't require them to do much, or any, course work because they did all of that as part of the previous studies. Of course, if you then look only at the pure time spend in what is called the PhD studies, you end up with a shorter time in Europe.
Now you can of course go deeper down the rabbit hole and ask why European universities expect PhD students to have a master's degree first. The reason for that is mostly historical - around here, we often didn't even have Bachelor's degrees until the above-mentioned Bolognia process. What happened as part of this process was that decision makers ended up deciding that pretty much the first three years of the old diploma studies became "the bachelor" while the remaining two years became "the master". Of course, this reasoning led to the public opinion of somebody with "only" a bachelor's degree as a glorified college dropout. The universities implicitly also shared this notion, as there were never substantial motions to admit bachelor degree holders to PhD programmes in most universities. Slowly, the bachelor programmes are getting more profile as something better than just the first 3/5 of an actual degree programme, and consequently their public image also improves. Universities are nowadays also taking first tender steps towards making it easier for bachelor degree holders to start a PhD - however, so far, this is mostly targeted at making it easier for international students to enroll.
Important concluding remark: I am aware that a lot of the above contained pretty sweeping generalizations, which do not hold true everywhere. Specifically, Great Britain and Ireland already historically used a different system. However, I wanted to answer with something a bit more substantial than "systems are different everywhere".
I can talk about PhDs in the UK, I have less knowledge of the rest of Europe, but I know that it is not similar to the US.
In the UK, you specialize early in a subject, and your education is therefore narrow and deep. In the US, you specialize later, and your education is therefore wide and not so deep. It's changed a little (but only a little) since I was in school, but at age 13, I dropped all but 5 subjects plus math(s) and English. I chose the three sciences (chemistry, biology, physics), geometrical and engineering drawing, and French. If you chose not to take any science at age 13, you were not going to be studying any science at university (I think this is changed, so you are required to take at least some sciene). Hence I have studied no humanities since I was 13. At age 16, you reduced again to three subjects - you are interested in science, people typically studied physics, chemistry, biology, or perhaps swapped the biology for maths. When I teach psychology undergraduates, health science undergraduates, or health professionals who are taking postgraduate courses, they will typically have done no math(s) since the age of 16. (This is a challenge, as I teach them statistics. If they knew anything about algebra, they've forgotten most of it. They will deny ever having been taught calculus [and that's true, they probably have never studied it]).
At 16, I chose biology, psychology and environmental science. (Env Sci is, or was, essentially applied chemistry and biology, with a bit of geography).
In the UK, you go to university to study a subject, and that is what you study. There is no concept of picking a major. If you want to change your major, you usually start again. (In my first year, I studied two subsidiary subjects for 50% of the time, after the first year, I did nothing except psychology courses.
When I graduated at age 21, I had a degree in psychology, and I'd been studying psychology for 5 years (and psychology had made up almost 2/3rds of what I'd studied from age 16).
The PhD has also changed, but in the UK at the time, the purpose of the PhD was to write a dissertation. That was the only requirement. In the US, there is the idea of PhD-ABD - all but dissertation. In the UK, this would make no sense, there is no requirement for a PhD except for the dissertation. You start, and on day 1 you work on your dissertaion. On day N (where N is quite a large number) you submit your dissertation, and you're finished. This is changing, or has changed so that there is a coursework requirement for a PhD; but in the US people talk about taking courses in departments outside their PhD subject. This is very rare in the UK - you take courses offered by your department, and you take the courses you have to take, no more. British PhD dissertations are considerably longer and more substantial than American PhD dissertations.
In comparison to an American student, a UK graduate in (say) psychology seems to know more psychology. But they know a lot less other stuff. In the US, it seems (to me) to be common to do a master's degree (or even a PhD) in a subject that you did not major in at undergraduate. For example, I've known people with a degree in economics or sociology who take a master's degree in statistics. This would be very rare in the UK, you would simply be too far behind everyone else on the course. (Many years ago, I applied for a master's course in applied statistics (an early online course) - I'd published papers on statistical methods in psychology, and had a PhD on statistical methods in psychology; I was rejected because my background was unsuitable.)
I think the goal is to get people to do the work of (approximately) 4 years undergrad, plus 2 years Master's work, plus 3-5 years of doctoral-level research. You can either lump that into 3 explicit degrees, or you can lump the latter two into one degree and just do a Bachelor's and a PhD. The overall amount of work is not that different. Not all people will get this equivalent level of training, but lots more programs have this level of required work than the simple US/EU divide would suggest.
Some folks (many? most?) in the US that enter a PhD program directly after their Bachelor's degree are eligible to pick up a Master's degree along the way based on completing the required coursework for the PhD. Some just don't bother to fill out the paperwork. I didn't. In the end, it doesn't really matter. Additionally, if you enter a US PhD program with an appropriate Master's degree, you can almost always short-circuit the initial coursework requirements and go straight to research. I think it's probably less common to do this because it requires 3 college applications and maybe more moving around than does staying at your first graduate institution, but some people do it.
The idea that European PhDs are uniform is misleading. Similar, there is no single US PhD. The differences in PhD programs, stem from differences that develop during primary and secondary education. For example, in the UK students begin specialising during their "A levels" and the undergraduate degree is a highly specialized 3 year degree. In the US, secondary education and the undergraduate degree include more breadth.
The teaching responsibilities in the US and UK are also different, with there being less off topic advanced level teaching in the UK. This means that post graduate course work, is to an extent less important, since you will not likely be teaching that material. For example, a CS researcher teaching in a small US EECS department might be required to teach undergraduate signal processing, but this would almost never happen in the UK. Having taken a graduate level class is really helpful for teaching undergraduate level classes.
Finally, there are cultural differences. In the UK, there is more pressure to get a job. In the UK, many people opt to skip a research intensive post doc and instead go directly into teaching intensive positions with the hope of switching tracks later. The funding models are different also
TL;DR They are different, because they are different.
There is a lot of diversity in American universities and a lot more in Europe. I have studied in four European countries and all of them had different PhD award requirement and average PhD duration, course work requirements, etc. A sweeping generalization would be that European universities require a master's degree, and PhD on average takes three years and in the US you require a certain number of credits and a bachelor's degree.
In reality, there is a lot of variability. Some American universities do require a master's degree before a PhD - e.g. doctoral programmes in engineering at UIUC. But many others admit you to begin a PhD and then expect you to either get a master's degree along the way fulfilling course credits or allow you to transfer those credits to PhD directly.
Interesting fun fact - several American and European universities have admitted anyone who passes the entrance exam to a doctoral programme - even if they didn't have a bachelor's degree. What they expect, to award a PhD, is just contribution to the field and proof of competence as a researcher. But now we have institutions that enforce credit systems, quality control and so on, so there are these hard requirements at some level subject to many things imposed by institutions based on country, credit system, university, discipline, department, PhD award committee and even perhaps professors.
These limitations now exist because there are way too many institutions and universities now, and I won't be surprised if there is someone out there who is doing a PhD in this very subject. If you find them, let me know. :D
European countries agreed a few years ago on having all similar official durations for PhDs (3-4 years). See https://en.wikipedia.org/wiki/Bologna_Process.
In all european countries starting a PhD requires having completed 5 years of study (european master degree), while in US it is in theory possible to start a PhD after 3-4 years of study.
TAship is less common and far less stringent in most european countries than in US.
In Germany and many other European countries, historically, a PhD was not enough to get a professorship (whereas in the US a PhD is required for associate-professor positions, I think).
In many fields, PhDs had to apply to the tenure committe, and then were allowed to write and submit a "Habilitation" paper or internal report. After acceptance, then you could apply for a professorship, and be tenured.
So, historically, often:
(Professorship qualification phase duration in Europe) = PhD phase + Habilitation phase
This requirement has been "reformed away" since the 2000s, and often been replaced by "junior professorships" (multiyear fixed-term contracts).
Local traditions and requirements with respect to "Habilitation" strongly differ. in some fields this still exists; and at some universities, it has been "reformed away" even earlier.
For "University of Applied Sciences" (Fachhochschulen - less prestigious but still pretty good universities), there was no such requirement.
