Because then they simply wouldn’t exist. If they did, interest rates would be stratospheric, with most of the money coming from the successful borrowers subsidising everyone else.
Sure, the student loan can only go to a school but neither has any collateral which makes them the most similar loan products. So go get any 18 year old to try to get a $20,000 personal loan without any credit and see what the rates are and if lenders would even give it to them.