There are good reasons Americans aren't getting post graduate degrees in STEM, I know first hand. I wrote the first-of-its-kind automated engineering graduate application system and sat down with the Graduate Admissions Committee. They told me first hand, their process is broken. It was flooded with so many foreign applications, they simply couldn't process them all. At the time, they had a directive to prefer US citizens, but they simply couldn't find them because they were lost in the tens of thousands of foreign applicants. They needed to rely on recommendations from existing graduate students and first hand experience from professors, which would produce the result you would expect: The graduate students will always recommend friends, family and people from their own county. Now we have decades of this culture to the point where a nearly all the professors themselves are foreign born and have deep ties to overseas communities and they are making the decisions about who will study under them.

Don't give me this speculative bullshit about there not being enough citizens to fill the universities, the competition is simply too great and racism (whether explicit with DEI, or implicit with nepotism) is a significant hurdle to enter most programs in public universities