A very different culture, university sport is THE feeder system for the likes of the NFL and NBA and the levels of investment in college sport are astronomical. You get multi millionaires and billionaires, often alumni, helping fund teams. Jimmy Haslam, for example has donated tens of millions of dollars to Tennessee University. Oregon has received around a $1bn in donations. Some universities now give out multi million dollar deals to players. Then you have the TV deals and revenue from merchandise, not to mention the ticket sales (for a big, non championship game, minimum ticket prices will be c£200).
College teams are often based in different cities to NFL teams. Thus, for many of those who live close to the colleges, it is the highest quality American football they can watch in-person without having to drive several hours. You also have alumni attending games as as the current student population.