Maybe my perceptions were wrong. I went to a Swedish university, and I suppose there is a lot of research going on, but I would guess there's not nearly the same amount of money in it. The US also tends to attract the smartest people, while Sweden is less attractive (not being an English language country) and with less money for doctoral students. Also, international rankings seems to favor US universities, probably because they produce good research, not that the education is in any way superior.