Seems to happen to most top universities, or is there some context I'm missing?
Fun fact: When I applied to Cambridge I was interviewed by Mary Beard.
Either due to corruption or, even more likely, institutional biases.
Malcolm Gladwell has a few episodes of revisionist history that looks at some US universities who fall into this trap.
Even with all that said; the chances of a selection process for 17/18 year olds being able to correctly select the best future adults in their field is very low.
Another good example; the number of young geniuses or prodigies (whether maths, chess, acting etc.) who don't make anything of themselves.
https://www.quora.com/What-are-some-claims-by-Malcolm-Gladwe...
It's very hard to predict from a half hour interview precisely who will do well in future, and in fact some people might do better having gone to other universities. Oxford and Cambridge aren't the best choice for everybody - I'm sure being a bigger fish in a smaller pond suits some people better.
I read a study a while ago (sorry Google didn't find it again) that said a lot of exceptionally gifted people just don't apply to elite universities. Instead, the main characteristic of applicants tends to be that they are extremely driven.
Drive tends to not correlate well to long term success without talent as a companion. Therefore a lot of people at Cambridge (which I think was the example) tend to do no better than the average in later life (notably wasn't true for PPE grads obviously)
that's a complete sentence already
Top universities hiring professors can pick any they want.
But also discovering a passion for academia can occur at any institution, reaching the pinnacle of the career is an enormous commitment.