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.