Likely because they are still in college, and China would rather want the leaders in their fields to work for them instead, and also teach the newly graduated to take it from there?
Also, literally every student I've met in China wants to go to the US. Undergraduates want to do their Master's there, Master's students want to do their PhD, I guess PhD students want to become professors at a US university.
Yea that's also true, in Asia, the western education is perceived as far superior to theirs.