> if their research is so important

Because it is not. US tenure positions are already saturated with far more applicants than available positions. There are still a lot of top tier chinese professors at US universities. But, the US is skimming the cream of the cream of cream. Some inevitably fall through the cracks.

Also, US universities expect their profs to teach. I know for a fact, that grad school rejected professors for not having adequate English proficiency for teaching. These same people probably excel at teaching in Chinese back in China.

It is also a culture thing. Sometimes US universities want to recruit talent that fits in with their culture. Sometimes, bad culture matches for US universities, might just be good culture matches for Chinese universities.

I think the unfortunate truth is that US academia job market is already highly saturated.

Look, a professor may need 5 to 10 PhD students, but there won't be so many positions open when those students graduate. In the past physics and biology graduates will try to stay as postdoc or research assistant as long as possible. Or find a position in industry. Nevertheless, this is just not a sustainable model in the long run. Eventually it will come to a point where more and more of them will go back to their home country.

there aren't enough positions. I work at a large internet company that employs literally thousands of PhD scientists (PhDs in fields like astronomy, bioinformatics). Many of them trained to be professors, then found that the market was full. They make more money and work less as data scientists/programmers/product managers/people managers, but without the level of satisfaction they might get from being a professor.

It is unclear whether having many more scientists would increase the rate of novel discoveries or of general useful information production in a country like the US. On the other hand, China intends to be a full member of the international scientific community and needs publications in prestiguous journals to get there.

Because their research is not important. If it were, academia would pay more, and have better working conditions. If it were, immigration to the US would be easier[1].

That everyone is wringing their hands over this issue today is quite telling. The people doing it aren't interested in advancing research - they are interested in using public money to make privately capitalizable discoveries.

If a single penny of public money was used in research, the results should be public. At that point, it doesn't matter whether or not that research ended up going to China.

[1] Yes, it's possible for researchers to immigrate to the US. But it's not exactly easy. It's a mountain of beurocracy, stress, legal fees, arbitrary restrictions on travel, and a big drain on your time. Combine that with all the other costs of permanently moving to another country, and you shouldn't be surprised that a lot of people opt not to do it.