I also wonder how much of that growth is from high school students being told to go to college above all else, and how this graduation increase corresponds to enrollment and graduation from vocational schools.

I do think your point is the big one: people could go into college and come out with knowledge in the so-called soft skills, like philosophy or literature (areas that don't pay well but are vital for a society to understand itself, if nothing else). Why society doesn't value teaching and similar jobs as much as it does other industries is left as a debate for another time.