Given that much research is from American public financing, why pay foreign grad students to perform research when salaries could be raised enough to attract US resident students.
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
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.