The collateral is a claim on future earnings, isn't it? In the UK, the student loans company gets back 9% of your salary post graduation and you cannot legally opt out until the loan is repaid provided you earn over a minimum threshold.
You can't squeeze blood from a rock. Sure, you can say 'hey let's garnish this guy's McD wages for the next 25 years' - but the collection costs are almost as much as you can get your hands on. Just not worth it. Well, only at very high interest rates, so that the good ones make up for the bad ones.
I'm not suggesting collateralized student loans, I agree that's difficult without a durable good. I'm suggesting a similar risk-based approach for determining the size of the loan. This would cut down on the number of people who have $200k in student loans and a MFA which generates no income.
The difficulty of getting loans for degrees which are less likely to be able to generate significant income might even lead to lower tuition at non-STEM schools (hey, a guy can dream)
Risk for loans is low(er) for material assets (because they can be repo'd), which is why interest on loans for them is so cheap. Education loans are priced 'correctly' by the market (i.e., very expensively), the 'problem' is that people consider them different from material goods because they're the best/only way for social advancement for many/most people (which I don't disagree with, I'm not making some sort of moral statement here, just explaining the mechanics.) So then 'they' (as in, some amorphous group of voters and politicians) want the government to step in, which creates disincentives all around, yadda yadda yadda and then we find ourselves in the situation we're in now.
So, to come back to your question, why don't we treat education loans the same as other loans - because 'society' doesn't think they're the same, for moral/equality/social mobility reasons. It's really as simple as that.