So what is high school for?

Also does the above apply also to the most selective and renowned institutions, or only to community colleges?

High school is preparing to be able to do a college education? A bachelor's isn't that trivial most of the time.

I'd say applies to most colleges, of any size. Community colleges tend to be more like trade schools

In Europe high school is when you get a generic education, and in college you choose your topic. So if you are a languages major you'll have no more math, if you're a STEM major you'll have no more history, if you're a philosophy major you might have logic and some history but no physics or linear algebra, and so on.

I studied computer science (M.Eng.) and I had to provide an English certification as a foreign language, but that was it. The only non-computer science, non-math, non-electronics courses were one semester of chemistry, one of economics and three of physics (which you could say is related to electronics though). So basically 15-20%.

Having to study more history or biology would have been a huge waste of time in a CS university, and I say that as someone who did more Latin and Greek than math and physics in high school.