It seems like it might be a good business to offer to refinance loans for, say, CS students at top-tier schools at a lower rate than the government loans.

Ideally that would raise the default rate of the government loans and push up the interest rates of the arts students. Right now the STEM majors are subsidizing everyone else if the interest rates are the same.

I thought that was a pretty good idea so I looked to see if it was legal and lo and behold: https://www.upstart.com/

A startup exists which refinances loans based on college major and SAT scores

Why not just progressive taxation?

Don't predicate the rate on a hypothetical earning... Base it on actual earnings.

The interest rate is acting as a market signal to pursue majors/professions that would lead to higher actual earnings.

Sounds like a recipe for an astronomical default rate.

It would theoretically lead to a lower default rate, as it would dissuade "art history" majors from pursuing that low-paying profession. As it stands now, the people who pursue higher paying majors are subsidizing those who pursue the lower paying majors. You can't seriously say that an engineering major is the same risk as an art history major. Then why should they pay the same interest rate? You aren't going to claim that everyone should pay the same mortgage rate, right?