China provides many opportunities for its citizens to study abroad and expand their horizons, and the US is happy to take that money and nearly free brain labor. But when the fruits of all that training is ripe to pluck, the scale flips and now it is a fight between those two countries to retain the talents. And the US is losing badly.

Maybe China is playing it dirty and forces these scientists to come back. Or maybe it is simply that the US immigration is so hostile they don't see a future here. One of the main reasons people tried to go here is because of a chance of a better life. But when the naturalization process is so grueling and drawn out, they just don't bother anymore. Especially when China in big cities is providing pretty high standards of living to its citizens. Sure, "freedom" and "human rights" are something but frankly, those concerns are not relevant when we are talking about the upper class Han people. Stereotypes of rich Asian international students exist for a reason and that is because many of them come from rich and influential families. To these people, as long as you don't stir things up politically, they will feel safer and enjoy more rights in China than in the US.

Lots of US citizens do not understand the pain of immigration. It is in every way a demeaning and torturous process. You get treated like second class human beings and are forced to fight and prove you deserve something others were simply born with. There must be an incentive for these people to endure that kind of process. And for many Chinese whose homeland in every way a direct competitor to the US, there aren't enough reasons.