Demonstrating the ability to go four years without getting kicked out of school for discipline problems and/or getting incarcerated. The GED is sold heavily to inmates as a way to get back on their feet after they're released, so the people who end up taking the GED are preselected to have severe self-control issues. Learning HS material doesn't correct this: the reason nobody wants to hire them is because they were in jail, not because they don't know trigonometry.

Education is about learning to follow the rules of an society: coming to school or work in time, sitting still and quiet while being bored for several hours. You know, the important skills you need to be a good little worker bee.

I would say a number of things, but one big one is to simply show that you're the kind of person willing to jump through various hoops to get where you want to be.

Actually, I think at the undergraduate level it's even worse (you still learn a lot of generally useful things in GED/high school). I would guess that what most people actually learn at the undergraduate level is completely unnecessary for their eventual careers. There are certainly exceptions, but it's a good rule (consider this: why do you need a bachelor's degree before entering med school? We invented a "fake" major for this requirement--premed--but objectively, it's completely unnecessary)