It doesn't.
But airlines don't hire pilots, they hire employees.
An undergraduate degree makes a person a better employee, not a better pilot. They've shown that they can accomplish a (minimally) rigorous academic program and do it while interacting with a whole bunch of people from a whole bunch of backgrounds, do it on time, on budget, and without pissing off the wrong people so as to get kicked out of school.
It's a good predictor for how that person will behave as an employee, and although it's just one metric, it isn't dispositive.
Airlines don't hire the guy that can shoot the best single engine ILS approach, they hire the person that they think will make them the most money. Making the most money requires the best employees.
Also, you have to look at it from HR's perspective. Everybody in HR will have, at the least, an undergraduate degree, and quite possibly a better education than that. How do you convince HR that they should give you the keys to a $300 million aircraft if you can't be bothered to follow directions and get an undergraduate degree? That has nothing to do with being a good pilot, it has to do with being a good employee.
So what you are telling me is basically that, running a team of guys, following the direction of management and making it to a role as a lead, passing all my certification tests in that field, learning and being sucessful at a trade, and being solicited for positions in that former career field doesn't show the same? I move to Florida from California because my resume was online and I was solicited for a "Lead Technician" position. My team consistantly made more money for the shop than the other four teams, and we had less comebacks than the other teams. I would argue that a four year degree, while it may do some of what you speak of, would show the same as one who has that kind of background. Following direction is just as important as giving it. Being a supervisor requires following direction to get to that level, as well as being able to give it once you reach that level. Both good attributes to have in someone in control of a $30 million jet($300 mil?).
As for what HR does, I couldn't care less what that job requires for entry, it's not my part of the industry.
Another good predictor of how well someone will do is their previous work experince. Did they jump around from industry to industry? Or did they stick it out in their previous one, or the one they are currently in, and make something of it? I did, and I am. That is also a good predictor. I could also buy that argument,
if they didn't require me to list my previous X years of employment history. But they do. And employment history shows that you could not only follow direction(or not), but also that you were good at it, because you didn't get fired("are you eligible for re-hire" and "have you ever been terminiated or forced to resign") .
I've know pleanty of guys who do have the degree that can follow direction until they get out on the line. I dealt with a Riddle grad where I was last that was a great test taker, but had the people skills of a wild boar and has been jumping around from job to job because he runs his mouth, refuses to follow direction, and runs customers off. I've also know more than a few "cowboys" who after a few years, finally get their degree, and it doesn't change them in the terms you are speaking of. There is one in specific that I can think of that is a member here who will remain anynomous. At least pubically.
Anyone can get a four year degree whether they follow direction or not. It may take them longer, but they can still get it. I've known guys who are ASE Master Tech's, that couldn't fix their way out of a wet paper bag, with the aid of a service manual and a Tech Hotline to call.
IMO, it's used as an easy metric to weed out people and make HR's job a little eaiser because I am certian that with the flood of resumes that some companies are getting, it must be a real pain in the ass (so glad I can say ass here!).