Money. Family problems.
I knew a girl who was in the Mensa category. She didn't go because she couldn't tolerate being near people. Such a shame.
Why don't many smart people go to the top 10 universities?
If they're REALLY smart ... They realize those universities are more about "prestige" than education.
It doesn't matter to most employers where you got your degree from, as long as it's an accredited school.
So why pay a lot more to attend one of those "top" universities ... Unless you need their research facilities?
Which university you choose to attend is based on your personality, your interests in general, your intended field of study, your interest in gaining a certain network of people, and occasionally, where you plan on living after graduation if you have a significant preference. (Some companies prefer graduates from universities in their region of the country.)
My father told me when I was a teenager that I could go to Harvard if I wanted to. I had absolutely no interest in going there.
For me, that was an excellent decision.
May be they find college mediocre?
Truly smart people put a higher premium on their desired field of study and college experience than on poll rankings from strangers.