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.

Being on time, and attendance, these seemed to get emphasized more than actual performance when I was in high school. This reflects how some of my worst jobs were, too.

Far be it for me to defend US high schools, but being on time is very much a skill that people lack - and it so annoying, not to mention a sign disrespect for the other peoples time, to be later for appointments.

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)

I've never heard of or seen a premed major. I've heard of premed students getting degrees in bio/chem/eng. Just like there is no prelaw degree. Just phil/polisci/engl degrees with courses suited for taking the entry exams.

It's pretty common. For example: http://www.science.psu.edu/premed/premedicinemajor.html