Well we know money drives the world we live in. Number one, most of these college stadiums (even for private schools) receive state funding for their construction. But so NFL stadiums. The real disparity between NFL stadiums and college stadiums are the alumni (students) and former athletes who pump money back into their school. I mean, Carmelo Anthony just shelled out 20 million to have a brand new basketball practice facility built at Syracuse.
I'm pretty sure at Miami, players like Ray Lewis, Warren Sapp, Clinton Portis, Ed Reed and others give money to the program. The same goes at schools like Michigan, Ohio State, Florida, Florida State. Especially for the state-run schools, they just have bigger coffers than the privately owned NFL teams.
Also, pro games are expensive as is. So if your gonna charge $100 bucks for a ticket, the seat must be scarce. Which mean the stadium isn't holding 100,000 people, more like 55000.