It’s because graduate or professional schools judge success individually. It’s not about where you go, it’s what you do with the opportunity you have in front of you. If graduate schools started to create multiple standards based on school prestige, they’d be sued. Not to mention, it would be a huge headache to enforce. Imagine the headache if undergraduate schools had to rank high schools. That’s why you have tests like LSAT, GRE, and MCAT.