if education in the US is so crappy, why do foreigners come to our schools when they are CLEARLY oftentimes?

more well versed in the subjects than their fellow American counterparts? it makes no sense to me. like, there was this post i was reading on a math forum and there was this kid who was in high school but aspired to major in math in college even though he's not that good at it, as he says. there was this asian guy who came on there telling him "don't do it!" if you can't do real analysis yet then don't bother, you'll SUUUUCKKK! i took that class in HIGH SCHOOOOOL cause i'm so SMAARRRT!!! WHAAAT your high school doesn't offer that course!?!? then gtfoh!" (keep in mind that real analysis usually is an upper level college undergrad course but whatever) i'm paraphrasing, of course but this really seems to be the attitude that many foreigners have. i know they realize that they're more further along in their knowledge of the material than americans in those same classes, so why do they come to american universities if their own schools have apparently taught them so well? isn't that like a downgrade? i'm confused!