There need to be a few things that allow a corporation to separate people out of a pack. A four year degree does that. Does it eliminate some great candidates? Yes, but it also gets rid of a bunch of people that would be trying very hard to prove their smarts.
I saw this first hand many many times at my former regional carrier. At DAL you know that 99,99999% of the guys have a degree, so there is absolutely no one upmanship going on. Yes, I mean it. A Captain knows that you have X qualifications and one of them is that you are able to complete an accredited (for????) year degree. It tells them that at a min you can complete a college approved curriculum. Does it make you smart? No, but it also means that you are not a dolt.

DAL likes advanced degree work. Most of the guys I fly with have post graduate degrees if not PHD's. It is not often that I fly with a pilot that does not have an advanced degree and or a business on the side. Most of the guys here are too smart for their own good.

Why we do it? Just another box to check and we have been doing it this way forever. Will it change? Probably not as we promote from within!