Well, look at YC's stats on success rate vs. founder went to an elite school.
- VCs biasing toward founders with an elite school on their resume
- Elite schools offering more opportunities for building a start-up (Stanford, CMU, etc. have separate funds dedicated toward investing in student start-ups)
- Those who go to elite schools usually come from a place of privilege allowing for higher risk opportunities
- Those at elite schools have more connections to get their foot into a VC's door
and so on..
The problem with both situations (VC/entrepreneurs and employers/employees) is that there's a cost associated with evaluating a candidate.
I don't think anyone doubts that good opportunities (and candidates) exist at all at lower tier schools, but it seems very likely that there are fewer of them.
If your resources are already constrained by something other than the number of opportunities you can afford to fund/hire (e.g. time to evaluate/interview), then it doesn't make sense to expand your search if it means lowering your success rate.