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.
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.
that's a complete sentence already