You'd think students would be incentivized enough not to dig themselves into thousands of dollars in debt, but students are also incentivized by the wishes of their parents and the promises of a good future. Incentives don't have to be in someone's best interest to be motivating.
Universities don't care if you default on your loans, they've already been paid.
If they're dischargeable, rates will just go up, or the government will have to subsidize them further. 17 year old kids aren't choosing whether to go to college based on whether the rate they get on their loan swings a few points in either direction.
If a politician runs a campaign on “reducing student loan assistance” they will have committed political suicide and it won’t change the minds of any mother pushing their 17 year old son to go to college.
The universities were happy to take the money, from the elite-but-useless private schools to the underfunded but more practical public engineering colleges. Only changing the incentives will change the results.