If I were the Big 12 or AAC, I'd want to keep the exiting members as long as possible. If they are worth money as members, keep that money coming in. If they generate payouts that stay with the conference, keep them hostage as long as possible unless they pay such an exorbitant exit fee that it leaves the remaining schools set for a while.

If their new conference wants them badly enough, they'll pay a lot more to get them. If they bring a lot of value, it may be worth it to them.

By forcing them to stay as long as possible, it keeps their athletics department in a kind of lame duck limbo status and can affect numerous recruiting classes, while still benefiting the original conference in the short term. And it also denies the new conference it's new members for as long as possible so it helps keep competing conferences revenues in check for a tiny bit longer.

If the exiting members are going to increase their revenue a lot, and in turn, use that money and status against the old conference members, its reasonable to make that hard for them. If the increase will be large, its smart to make it a long term gamble, and have at least a decade before they even begin to come out ahead.