I think a portion of it is that if you judge based solely on academics then you will only get a certain type of person who is focused on scores in academics. You could easily fill your class with the top X% of scorers but that won't necessarily make an interesting group of graduates in the real world. It especially won't make an interesting group of people to interact with. I don't think I'd go to that school even if I had been admitted.
I think part of it is that they're hoping to find people who will end up being important people in the world. Not necessarily just those who perform well academically.