U.S. undergrads have to complete "general educational requirements" that are taken care of in high school in practically every other developed country. Why does that happen? Because U.S. colleges don't trust K-12 to provide a satisfactory education.
We do not trust the average student, however access to universities in the US is less gated on ability or achievement than most of the nations you're going to be comparing it to.
All other things being equal (including quality), high school is actually better than college at doing the "comprehensive education" thing. College level gen-eds are almost universally reviled as a pointless box-ticking exercise that gets in the way of specialized education. This particular dysfunction has effects even further out; U.S. college education pushes things out to the grad school level that are elsewhere part of the later years of undergrad.
