That, and there is virtually no way to accurately measure the default risk of those taking the loans.
It would be very interesting to live in a world where your choice of major affected your loan rate. For example, as a political science major you would not be nearly as employable as a computer science major, which would translate to more risk of default. This would ensure that only those who are serious about a political science degree would pursue it, and at a big picture level things would shift so that people would think about the real value of their education much more carefully.
Imagine a situation where you are looking at a list of majors, and next to each item there is a percentage score, which is the loan rate. For electrical engineering it might be 2.5%, whereas for art it might be 9%.
But then you would also have to consider the risk that someone who is not qualified for or interested in electrical engineering would most likely drop out of it. So the formula would also have to take into account the person's background. If they scored high on their SATs and Advanced Placement tests for math and engineering, the loan rate for EE might further go down to 2%. For those coming from a non-technical focus in high school, it might go up to 3.5%.
Hmm...