American exceptionalism is a very simple thing: USA is the only country in the world in which right and physical ability of private citizens to invest into productive economic assets is a necessary cornerstone of existence of the nation itself and thus cannot be put at risk, in spite of any imaginable political dynamics. Society as a whole and political system in particular are deeply invested (pun intended) into dependence on this unique feature and no political party of group that threatens it, can survive. America also does not depend on anything external, being self-sufficient in everything, while everyone strongly depends on their ability to trade with it. It is also, due to pure geography, immune to invasions. It makes it attractive to investment both foreign and local. That simple. It's simply a large, self-sufficient society based on capitalism.
China may be an economic giant, but what's in it for me? Show me a way to invest into it and benefit from their growth? I just can't do it. And neither can the Chinese: they use all the tricks they can invent to siphon their money out of China - and buy US stocks.