The bottom-barrel students never end up in American colleges, after all.
The chief difference is cultural - in Asia intellectualism is celebrated and demanded. Parents don't push their kids to football, they push them to academic achievement. Not being able to make it into college isn't worn as a badge of honor, it's downright shameful. I have personally known people who've shipped their kids away from home to some backwater third-rate academy, just so they can hide the fact that the kid couldn't get into any school locally.
Beyond parental pressure, peer pressure also largely pushes people to academic achievement. Having high marks won't get you beat up - in fact it commands respect (within reason). Having poor marks likewise isn't cool like it is here, but is similarly embarrassing.
Keep in mind though, that this has some consequences that academics who opine about the lack of American work ethic consistently fail to mention:
- Appreciation and participation in the arts, both as a college study, and as extracurriculars, is ridiculously low. People who study the humanities are looked down upon, I even know one guy who got disowned for it.
- Social integration takes a backseat, often to disastrous results later. In Taiwan is it not irregular for a middle school child (age 12 or so) to be at some form of school for 14 hours a day. There's literally only spare time to eat and sleep (and sometimes not even). Socialization is minimal, and add this onto the culture of living with your parents well into your 20s, it produces some seriously socially inept people.
Japan is already dealing with these consequences - look up "hikkikomori" if you're interested.