Because scoring 50 on a shitty defense is easy. Giving up 36 to a terrible offense isn't. They haven't played a really good defensive team all season, but they continue to give up gobs of points to atrocious teams. That's a recipe for disaster.
How is Cincinnati threatening Ohio State? Are you fucking serious? Cincinnati being good happens as often as Papal elections. For every good season that UC has (i.e. every time they have a schedule loaded with patsies and a quarterback with a pulse), they have 5-6 years of being remarkably awful. Cincinnati is as threatening to Ohio State as Indiana or Northwestern.
I agree, it is impressive to win all of your games. That notwithstanding, it's less impressive to win all of your games when your opponents are barely over .500 as a group. How many ten win teams did UC play this season? How many ranked teams? You can dump on a lot of teams for playing shoddy schedules, but the fact of the matter is that UC has the weakest resume of the top ten by a fair margin. Even Boise State has a better win than UC. So great, they won all of their games. Fantastic. It still doesn't mean they're any damn good.