Aha! That's the mistake that so many people have made in analyzing conference realignment. The power conferences don't want to force Notre Dame into a conference. Instead, each power conference wants to force Notre Dame into its own conference. There's a massive difference there. Sure, the Big Ten would love to have Notre Dame, but if they can't have the Irish, then they don't want anyone else (particularly the ACC) to have the Irish either. They'd rather keep them as independent. In turn, no one outside of Big Ten country, especially the SEC (who would lose relative power) and ACC (who could lose additional members), wants to see ND join the Big Ten that would create an even greater financial monster (which is bad enough in their eyes already today).
The upshot: each power conference individually would be better off if Notre Dame joined them specifically, but they all are better off collectively if Notre Dame stays independent. The downside risk of ND joining another conference is worse than taking the upside risk of attempting to "force" ND to join your own conference when they aren't guaranteed to do so. That's why you have never seen any indication, even from Jim Delany, that they would shut out independents from the playoff system. The status quo of ND staying independent is better than the upheaval from ND choosing to join someone other than your own conference. They'll all temporarily delude themselves into thinking that they can get ND periodically (even the Big 12 fell for it!), but once they sober up, they realize that it's better to leave well enough alone.