It's a giantass school, in a pretty big city. The reason for the extremely large and enthusiastic fanbase is because:
a) they're perennially at least decent, generally "good"
b) the stadium itself and the tailgating/gameday environment all over campus are some of the largest and most impressive in the college football world
c) there are so many students, faculty, and staff associated with the university that almost everyone has a family tie or close link, which is enough to become a fan for many
With USC being another perennially good team, and one that beat the shit out of OSU in their last meeting, this is a grudge match, and an opportunity for vengeance. I would argue that the (unpaid) OSU athletes on this team will have more fight and will to play at their highest level in this game than whatever NFL athletes play in the super bowl this year. For you to suggest that this game is only worth the money, hype, enthusiasm, whatever to stupid retarded OSU fans is baseless. You will see this when the campus area is overrun by not only fans but an ARMY of media people. It's not solely your standard "50 mile radius of Columbus" fans buying the tickets either, and when USC fans and OSU fans abroad are in the market for tickets the cost will get obviously be driven up. It's a big deal outside of just Columbus, and you're the dumbass if you don't see that.
You've got to be simply anti-OSU or waaaaaay out of the loop to not get the scenario here.