> One rationale that I heard that seems reasonable at face value is since you can't give back your education, that's the justification for not being able to declare bankruptcy.

It goes a little further than that. It doesn't really matter that you can't give back the education. It's that the loan is entirely unsecured - there's no collateral, but also the people who get student loans typically don't have any assets. So it would cost nothing to take out student loans, study, and then declare bankruptcy.