The same reason that employers don't give all applicants a standard test and hire the people with the best scores. The culture of a school is something that people think is important and you can't control that culture by accepting people based purely off of academics. That is why things like admission interviews exist and kids have always picked up electives in high school that would "look good on a college application".

This practice has certainly been abused in the past and you can argue it is still being abused, but I don't think that means that judging a kid based on anything beyond academics is a bad move for colleges.