When I was at Google, I was asked if I wanted to make a recruiting trip to my alma mater. I was excited until I found out they were talking about my grad school where I got my MS, and not UB, a large state school where I got my BS. I told them I'd be happy to take a trip to recruit at UB. I got mostly crickets back from that reply. However, I ended up getting signed up for a series where they had a panel of a few HR…
Compare that to the students I interviewed for internships from top 5 engineering schools. Among them was one of the best candidates I'd ever encountered, regardless of experience level. He came up with a unique (and quite good) solution to a problem I'd been using as an interview question for years. As someone who had done fairly well in state school, I was floored by how much more well-prepared these students were than I had been.
My story is anecdotal, and there are many factors that can explain what I perceived. But I suspect the top companies have figured out that they get best results in hiring based on a) recent grads from top schools and b) great resumes from experienced devs who maybe didn't start out at top schools & top companies.
On the plus side from a diversity standpoint, the students I interviewed from these top schools were both male and female, and from a wider variety of ethnic backgrounds than I usually see in the hiring pipeline.