Feedback loops for "Is this degree a good investment?" are slow because it can take 15 years or more between starting an undergraduate degree to reaching maximum salary (and/or fully paying off loans).
So you can only really report on how good value a course was 15 years ago - and in the intervening period, there's a good chance the academics, curriculum, price, industry and economy will have changed a great deal.
But if student loans existed in a competitive market, there would exist huge incentive for loan providers to do everything they could to really figure these things out. I completely take your point that this feedback would never be exact, but with federal student loans, the current feedback is about as worse as it could be. In a true market system, loan providers -- who would care a lot about being paid back! -- would require higher rates for schools with more dubious success, and would therefore tighten the feedback loop between school quality and student enrollment.
So you can only really report on how good value a course was 15 years ago - and in the intervening period, there's a good chance the academics, curriculum, price, industry and economy will have changed a great deal.