US jobs have to be the best paying in the world, as the cost of living there is also among the highest in the world, and the tax system there is probably one of the worst - if only because you have at least county level, state level, federal level taxes, and maybe a few sublevels and other kinds of taxes that have to be paid for and all have their different rules on what constitutes taxable income and what not. There is more to income and salary than just amounts of money.
After growing up and studying in a place, most people will stay there. A while back I heard that most people in this world (to the tune of >90%) get born, live their life, and die within a 20-30 km radius. Sure you always hear about ex-pats, people moving far away from home (I'm one of them), but overall most people stay close to home. That's the place they know, the place where their friends are, their families - for most people there is no good reason to leave home. Belgium is a fine place to live, I'm sure.
There are many Chinese that want to get to the US, but don't forget there are 1.3 billion Chinese out there. If just 0.1% of the mainland Chinese population wants to make this move, that's 1.3 million people queueing up - potentially adding 0.4% to the US population, and most of those end up in the university population, making the influx very visible.
In contrast, if 0.1% of the Americans is looking for a job in China (and really - I know quite some that moved this way out of their own free will, plus many that were asked by their company to do so), that'd be a mere 0.32 million, adding just .025% of the Chinese population. Barely noticeable.