In a 12 team playoff, those teams would be seeded against the #5 and #6 teams.
In reference to another post of yours, sure Michigan vs Ole Miss would get better ratings. However when, in a 16 team playoff, the #15 or #16 team takes a lead into halftime the ratings for the second half of that game would be higher than any college football game to date.
Another thing that happens in the bball tournament is that people will watch the team that beat their team until they lose. If your team loses in the first round, but the other team makes it to the final four it makes you feel better about the loss. The CFP "championship" game gets very good ratings, but there are lots of college football fans who have already checked out and don't watch. The "championship" game last year got lower ratings than the semi-final games. There is an idea that people want to watch the same teams play every year, some for the second or third time. However, looking at the ratings, many people go away once their team is out. The bball tournament gives every single D1 school a chance at the beginning of the year to be the champion. Fans get caught up with their team, and their teams chances. When their team is eliminated, tournament or conference tournament, people follow their opponents. Maybe root for or root against, but they follow. In the NFL, every single team has an opportunity at the beginning of the season. People become invested in their teams. Once eliminated in the regular season or playoffs, they will follow their opponents, to root for or against. In college football, the College Football Pageant will be made up of a select group of teams every year. In the four team system, there were 10-15 teams. Maybe one team in some years could break into the four, but it takes a LOT to do so. In the twelve team system there are maybe 15-20 teams that will represent nine or ten spots every single year. There will be lots of games with teams matching up for the third time that year. You might get 20 million to watch the "championship" game, but there are going to be a lot of people left on the table who simply lose interest before that game.