Because unlike auto debt and mortgage debt, the largest creditor for student debt is the federal government itself. Therefore, student debt is less a market transaction and more a direct public policy. https://twitter.com/scottelliotg/status/1362083634811981831

When 92% of student loans are federally backed, and 75% of undergrad students are at public colleges and universities, the amount of debt students have is a government choice, just like the amount we pay in taxes is or the amount we give in farm subsidies is a government choice.

By contrast, mortgages are almost all issued by private banks. The government can and does regulate these loans, but the government does not set how much a home costs and has only the most minimal influence over how much banks charge to lend money to buy one.