It seems like in most European countries a student has to take a 3 years B.Sc, 2 years M.Sc and then 5 years phd program while in the U.S the M.Sc is skipped but the length of the phd stays the same...
(Specifically talking about CS, but may relate also to other fields)
Is that correct, and if so, why is that?
Your understanding of the duration of PhD programs in the US is a bit off- the average "registered time to degree" beyond the bachelors degree for PhD recipients in the US is more like 7 years. See
[http://www.nsf.gov/statistics/infbrief/nsf06312/][1]
In recent years there have been some attempts to reduce time to degree, and the data reported in this link are more than 10 years old, but the situation hasn't changed very much in the last 10 years.
As @Thomas already mentioned the nominal length of a PhD program in Europe is often 3-4 years instead of 5 years, depending on the country. However, there is a big difference between how long policy makers think a program should take and how long it takes actual students to finish those programs... You can easily add a a year or more to each of the programs if you want to talk about the actual durations.
As to why such differences exist, the answer is just that such programs were designed in different national contexts by different decisionmakers who made different choices. Within the European Union there have been attempts to harmonise what a university program looks like. However, there is no supernational body of the same strength as the EU that encompasses both Europe and the USA. So differences between the EU and USA are likely to persist.
