>You have a giant pool of 17-18 year old kids with 0 assets and minimal skills.

No you don't. People who get charged $50k for Stanford (and other top-ranked schools) have enormous family income/wealth, and for better or worse the social contract of our system is that parents contribute to their children's education commensurate to their ability to pay. The only people stuck borrowing the full amount are those whose parents could pay but refuse to.

You do get stuck borrowing that kind of money if you go to an expensive but not very good private school (how much help is available to middle-class kids scales with quality). A middle-class kid borrowing through a state flagship school (who generally gets no assistance) is more like $100k.