Except it's not really attractive to many capable people in their right mind who wouldn't wanna go down that rabbit hole called academia with low salaries, uncertain future, require you to constantly look for a new job and move around every 2-3 years (typically year-based contracts with 0 guarantees about the next year), and maybe a very small chance of getting a real job at the end that doesn't have an expiration date from day 1 in your mid-40s with mediocre salary comparable to what a junior dev would get in a small IT company. The strain this puts on a family is a whole another dimension of the issue.

There is a reason why physics departments are populated with Chinese and Indian researchers: it still beats the opportunities and life awaiting them at home. For people in developed countries? Not so much.

BTW, an answer to your question is this brain drain: https://en.wikipedia.org/wiki/Human_capital_flight

Being a Research Assistant (RA) or a Teaching Assistant (TA) is not a job per se but part of ones training while in grad school. Whatever you're paid while performing such duties is just a stipend, and not a salary in the traditional sense.

So, no, universities are not going to hire Americans to RA and TA positions if they have no incentive to be in grad school.

Foreign students come here because they want to come here to learn. Being paid for RA and TA responsibilities is just a bonus. Professors reap the benefit of having some of the best minds from around the globe around them.

Top universities (Stanford, MIT, Ivy League) don't have a money problem, and they don't have a supply-demand problem. Prospective students would gladly join and work for free, be they foreigners or US residents. They do pay though, although it's not clear what drives their level of pay. Maybe one factor is that if they paid more, it could end up being counterproductive, as the post about f-u money that has been on HN recently can attest.