Not couldn't. Wouldn't.
I'll elaborate...
The exponential difference in media value for conferences between college football and college basketball is due in large part to the value of each sport's post-season. The money committed to the hoops tourney totally sucks the life out of the regular season. Now, keep in mind I'm not talking about this from a competitive standpoint. I'm not one of those yokels who says the hoops season is "meaningless". But from media value standpoint it's value is greatly diminished by the existence of the tourney.
The conference commissioners know this, and do not want to repeat that model for football. We can poke all sorts of holes in the "every game, every week" matters mantra about football. But from a media value standpoint that is absolutely true.
So, in a world where 4 or 5 superconferences are trying to extract every last media dime available, why would they devalue their core product? And that product is their ability to sell regular season rights, and carry those regular season games on their own networks. Once again, this isn't about the money that could be made from a playoff, it's about protecting the money that is critical to be made to ensure the viability of super conferences.
The one hedge is that if 4 super conferences emerge and decide to break away from the NCAA, I suppose they could form an alliance to create a 4-team playoff and share in that money. But there would be soooo many political ramifications of that approach (both in the splitting off and in the creation of a cartel) that I'm not certain we'll see that anytime soon.