For the same reason that UT/OU are supposed to stay for a few seasons, very few teams can contractually (without penalty) leave immediately.
Even if you add the AAC teams, they may be 2-3 years away from being able to join and may only overlap with UT/OU by a year or two (if UT/OU actually stay that long: call me a skeptic).
The Orphan 8 may have a hard time agreeing on expansion candidates and UT/OU may just vote "no" to any and all candidates meaning they would have to have unanimity on the candidates or risk not expanding...