It's all about odd and percentage.
<<10% UCB students may do very well later.
From MIT, it could be 25%-30%.
Also, a student at MIT, on average, has a lot more opportunities than a student from UCB, even while he is at school.
Of course nothing is going to stop anybody if he really motivates himself, works hard and smart. He can go to AZ State and do better than an average Joe from MIT. But what's the odd of the existence of such a person at AZ State vs MIT?
Again, MIT surely has a lot more people like such than AZ State :-)