Opportunity cost is real, and it's probably one reason why fewer US students (who can work in the US without worrying about visa or related issues) seem to be going into graduate programs. Graduate indenture is an unappealing prospect, even more so when you are looking at being worse off than your parents' generation.
Tuition coverage for grad students is a way for universities to take money from grants and pay it to themselves. (And for undergrads they often limit the amount that external scholarships can reduce the expected student and parental contribution, in order to pocket as much money as possible.)
It's still important to realize that grad students are cheap labor for research (e.g. from NSF's perspective if they are the funding agency) and teaching purposes (e.g. from the university's perspective.)
Tuition coverage for grad students is a way for universities to take money from grants and pay it to themselves. (And for undergrads they often limit the amount that external scholarships can reduce the expected student and parental contribution, in order to pocket as much money as possible.)
It's still important to realize that grad students are cheap labor for research (e.g. from NSF's perspective if they are the funding agency) and teaching purposes (e.g. from the university's perspective.)