Flip it around; how is it good to accept someone to college who does not have a good chance of doing well? How good is it to accept their money, probably put them in debt (even if not the catastrophic $100,000+ debt, even $20,000 debt can be quite a lot for a young person, and it's hard to get out of 4 years without at least that much), and claim very significant portions of some of the most important years of their life [1] if you already have good reason to believe they are unlikely to do well? (And remember we're judging lots of people so statistical bases are all we can use.)

It may not be the only criterion of use, but "do we have good reason to believe they will succeed in college?" is at the very least a critical bar that ought to be passed. It's can be the difference between college as the empowering, education experience it's supposed to be, and college as the meat-grinder wealth-extraction tool it can be when you feed in students who shouldn't be there.

[1]: Not necessarily "best", but being "earliest" in a world of compounding interest makes them quite valuable in their own way.