I think MIT has been specifically trying to recruit IMO winners - if you look at the Putnam winners recently MIT has been dominating. This wasn't the case until the past few years though.
It seemed like things began to change in the early nineties. I'm trying to track down who in the admissions office instigated it. I don't suppose you know?
At least when I was involved with math contests (early 2010s) it was more that there were a lot of ex-campers at MIT/Harvard, and that did a lot to convince me.
Po Shen did a lot to recruit students directly to CMU, but I'm not sure there was a similar direct line at MIT.
I'd also be interested to see this for Putnam fellows. It looks like there's been a start from Joe Gallian (https://www.d.umn.edu/~jgallian/PutnamCareer.pdf) which "part one" of this series links to.
For example see:
https://news.mit.edu/2024/four-peat-mit-students-first-place...