Because no program with a shred of dignity or common sense fires a coach 4 months after his season has ended (in the absence of major NCAA violations or criminal activity)
And since when do you ever hear of any school re-hiring a coach who screwed over the program with the NCAA? UT is not going to be the first to take that risk, especially on someone who hasn't made a Final Four at any level above D-II. Ohio State is never going to bring back Jim Tressel and he won a National Championship.
TPTB at UT may have been dumb enough to let Gruden slip away, but even they're not dumb enough to re-hire a coach that will have the NCAA sniffing around 24/7.
Fool me once, shame on you. Fool me twice, shame on me.