There are a couple of reasons why it's attractive to choose the MIT kids:

1) They presumably have other opportunities available to them that they gave up to pursue the venture. It's hard to gauge the passion of a founder if they couldn't have gotten a job at google instead.

2) Investors want innovative products that eventually turn into companies, which means they want smart people who have a cool idea and can tweak the model when needed rather than people who just follow a business plan. Snapchats business plan would have been horrible if they had one.

As for upward mobility, pedigree is not something you're born with. Anyone can go to MIT, Stanford, or Harvard (and they give out quite a bit of financial aid as well).