Oxford & Cambridge are very selective and entry is very competitive. Unusually for UK universities they hold a separate entrance exam for all courses. (Information about the extra admissions tests done by some universities and some courses: https://www.ucas.com/applying/before-you-apply/what-and-wher....)

Fun fact: When I applied to Cambridge I was interviewed by Mary Beard.

Any elite selection process has high risk for not selecting the best people.

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.