I'd say the same reason is fairly similar when it comes to University by the way.
We put a huge HUGE social benefit on education and a college degree. Not getting a college degree these days can often just make you feel like some hick who barely passed highschool. People without college degress just aren't as respected socially even with good jobs.
Everyone wants to think the best of themselves... so everyone wants to go to college, even people who shouldn't. It was just too fetishized by the older generations because back that college degree = genius. So again it's a product that plays to our vanities and core conceptions to ourselves. Then the government lets you go for "free" no matter how poorly you did in highschool. Not having to pay until after you graduate and have your kickass new job!
