It's really hard to learn the material, for one thing. Reading a history book or a calculus book by yourself is not at all the same as taking a class with a teacher. But there are free online classes with teachers if you have sufficient motivation.

But in any case if you don't have the official degree then you're still not in a good situation. Part of schooling is the socialization that goes along with it--living away from home in a dormitory, doing your own homework, not getting distracted--these are tests that come along with formal schooling. And then when you've proven that you can grind out a ten page paper regurgitating what the professor said, then you have been sufficiently lobotomized to work in the real world. That's what the degree paper shows.