Because coming out of a prestigious school doesn't make you good by default. It just means you were able to pass their tests well enough, whether it is from actual skill or playing the tests.

Just because someone studied biology at standford - or even computer science at Stanford - does not necessarily make one a good dev., that said, smart kids who get into such programs are likely higher in potential. Moreover, good devs usually have a few years under the belt.

They hire from a lot of places and do piecemeal, often shoddy consulting work, it's not a product company, there's nothing special about them.