Currently only about 10% of student loans come from private institutions. I'm not sure if I want the government saying "You can't have a loan to study X because you won't make enough money". I'm not sure it's even possible to say that, except for using average life time earnings that could vary wildly over time.

Teenagers applying for college usually don't have personal credit histories to be evaluated. Either we give out lots of student loans, or we don't.