There are not enough geniuses for the top universities to be filled with them. As someone who went to a “top” university (but not as top as those you mentioned)

There are not enough geniuses for the top universities to be filled with them. As someone who went to a “top” university (but not as top as those you mentioned) I think only a small percentage of students are really blindingly bright. Most are just significantly above average, get their work done, have at least adequate social skills, and come from wealthy families that pushed them towards selective universities. From there most students fall into “buckets” such as being an athlete, coming from a specific high school with highly wealthy students, playing a certain instrument, etc. Only a small portion get in from pure brains