Because there isn't a way to take back the knowledge/skills/information learned while in college. If you default on a home loan, then can take your house. If you default on a car loan, they can take your car.
What can they do when you default on your student loans?
The price you pay for that guarantee is that you can't declare bankruptcy. The government is propping up this industry and it has some parallels to the US housing before 08 (encouraging lending to subprime homebuyers)
Time does that anyway, you'd be surprised how much you forget when you don't use it everyday. Plus the knowledge get's stale. A university degree depreciates just like everything else.