The normally given reason is that the asset being paid for (education) can’t be taken back by the loan provider. If you get a mortgage and can’t repay it, the bank repossesses your house. If you get a college education and can’t pay your student loan, the bank can’t take back your degree or what you learned.

This is, of course, not any kind of valid basis for the current status quo and in a sane setup lenders would either have to be much pickier about who they lend to or require tangible collateral.