I suspect "clear a certain kind of bar" is the biggest part of this.

Imagine you're a recruiter at Facebook. You reach out to thousands of candidates, convince hundreds to come in for interviews, and almost all of them fail because the interviews are hard and test skills that need to be specifically trained. If you convince a Googler to come in for an interview, they probably have a nearly 50% chance of passing since they've already done it at least once.

The recruiters are not exactly incentivized to bring in people who will be great hires. They are incentivized to bring in people who can pass the interview.

My favorite econ professor at my (private) university often pointed out that our eventual diploma was far more about signaling qualifications rather than education; that you won't necessarily get a better education than at the state school down the road, but it proves you can play the game; get the good grades in secondary school, do the things you need to do to get into the university, get the grades, do the work. It's far less about what you actually learn or the 'quality' of the education, whatever that means.

I know when I see a fellow alumni of my almamater, I think, well they are probably not much dumber than I am.

Doesn't it also mean the opposite? This person was at a better company now this person wants to work here. Why? What went wrong? Did big company make a mistake hiring. Vs this person seems to be on the rise. Each company is a bigger and each position is more senior.