I get what you're saying though, being a typical jack of all trades high school overachiever isn't the best strategy for getting into MIT. I don't think that's because they don't consider personality though - heck one of the admissions short answers is "what do you do for fun?". I think it's actually because they are trying to avoid the type that does things just to get into college or just because it's the thing to do at their school. If you're an absolute prodigy then it probably doesn't matter either way, but if you're borderline MIT is definitely the top school that tries the hardest to take the kid that seems more genuinely passionate and not like a social climber.
MIT also does the best job of considering context of all the elite schools. If you went to a great private high school you better have taken full advantage of those opportunities, whereas most other elite schools will just see the better overall resume/prior education. This is pretty clearly reflected in the schools' respective demographics.