Bryan Caplan in his book The Case Against Education suggests that employers value college degrees partly as a signal of conformism, and that that's the reason college is so hard to replace.
(His theory is the college signals three things: Intelligence, discipline and conformism. It would be easy to signal first two things in some other way, but the very fact that you think you know better how to signal them proves that you're some kind of smartass who's inevitably going to cause trouble.)