To stay included.
They'll make the most money and get the most exposure doing this. If the SEC and Big 10 went off and did their own thing, I think they would make less money than they currently do, but they'd still make a lot of money. And I think this playground "you can't sit with us" mentality drives as much of college football's decision making as rational thought. So even though everyone would probably be worse off in this breakaway scenario, it's still a very realistic/plausible threat and that will motivate the Big 12 and ACC (and G5s) to go along with it. College football isn't just the power leagues vs everyone else. There's at least 3 tiers, and maybe 4, of conferences. The Big 12 and ACC aren't on the same financial footing as the SEC and Big 10, but they're a long ways ahead of the guys behind them. And there are probably a couple tiers in the G5 with MWC and AAC at the top and CUSA and the MAC at the bottom, and the Sun Belt kind of between. Eventually I think the top brands in the SEC and Big 10 wonder ask themselves why they continue to subsidize the Purdues and Mississippi States of the world. I think that is more likely than the current SEC and Big 10 breaking away from everyone else, and likely the next frontier of conference consolidation.
The only part that's objectionable to me is the guaranteed bye for the SEC and Big 10 winners. Them having more guaranteed bids doesn't really bother me. And I'd much rather have the Big 12 getting a couple bids to this dance than not.
All this highlights how unfortunate it is that there has never been a central governing body in the sport. 8 9 team geographically aligned conferences/divisions who's champions play in the playoffs would be so much better than what we're going to get.