I attended high school both in Europe and in the US (exchange programme), and my experience was that the US educational system is roughly "one step behind", timewise, compared to the European one. The academic level I experienced in the senior year of US high school was roughly comparable to my last year of middle school in Italy. From friends who came through the US college/university system, I also get the feeling that their first few years of higher education (the general-knowledge, no-major-declared-yet years you are wondering about) covered roughly the same range of topics and complexity as my Italian Liceo.
For example: the highest maths class I took in the US was Trigonometry (and even this was done mostly using a graphing calculator). There were harder classes one could choose, sometimes for AP credits, but you could also easily complete your HS maths requirements with an accounting class. In Italy, I had started calculus (non-optionally) years earlier. Likewise literature was very much at a pretty basic familiarity (rather than analysis) level and foreign languages were mostly films and vocab quizzes.
Now this is my own experience, and there were other aspects (the US HS was in a very rural location, and I did not pick some of the hardest classes as they were held at the local college and I couldn't drive). But all other exchange students from Europe had a similar experience and we all easily coasted on the knowledge we'd gained in our early teens, even with the language barrier.
As to why, I have a few theories, but I don't claim to have an answer.
For one thing, European education tends to stratify much earlier - in many countries, you choose at some point in your early teens whether to pursue a more academic, university-focussed path, or a technical qualification. To do this, you need to have a variety of schools available within range - much of the US is too sparsely populated for this. Stratification allows European schools to challenge the more academically oriented students at an earlier age and get them to University with a more solid background; it also means that important life decisions are made much earlier, and may be much more biased by factors such as family background and socioeconomic status rather than the student's inclinations and skills.
The focus on non-academic endeavours was huge in the US compared to Europe. Sports, extracurriculars, clubs, productions - all these had the same standing, if not sometimes more, than classes, and were allocated time proportionally. Even some of the classes were, effectively, clubs - like "yearbook editing". I am not making a value judgement here: do extracurriculars distract from academic pursuits, or provide a more rounded education? Is it a better idea to dedicate your teenage years to study? Sports? Art?
But a practical result was that US students reached university without a lot of the general knowledge that European students - especially those who went to the more academically oriented types of high school - had acquired at their age. The first few years of college/Uni provided that, for those who went.