Part of the reason it exploded though is there seems to be a bottomless supply of student loans, I am guessing because of how hard they are to forgive unlike regular debt. So many teenagers are encouraged to borrow whatever the schools ask by virtually everyone they know so that they can get their degree, and lenders will gladly give them 30k a year to study zoology or some other field with a rough job market.

The result is college is more attainable to people across the social stratum. And then degrees are less rare, commanding a lower premium while the people holding them have more debt.