So here's the thing about the NCAA March Madness vs a playoff in college football.
Say you do let in the Group of 5s, how long before the first matchup? It's usually 2-4 weeks depending on bowl game and conference championships. So Penn State draws Boise State for a chance to win and advance to a chance at the national championship and gets a full 15 extra practices to focus on preparing for them. I mean, you may have an upset but I'd expect most of the powerhouses in the Power 5 to lay the wood.
Because the big difference is other bowl games don't serve a purpose like the playoffs do. Coaches want the extra practices to benefit the team for the next season, but it really won't do much for them in the current season. "Rankings" don't mean much unless they are related to seeding in a championship playoff.
Basketball is just a different beast, it's beautiful and I love it. I love that undersized, less athletic teams can shoot the lights out on a blue blood and knock them out of a tournament. But "win or go home" in a game like football, I'd be shocked to ever see a true underdog upset someone big time.
Why? too much preparation time, too much at stake, too many players for one to make a huge difference. There's also nothing in-game that can separate smaller/less athletic teams from the bigger/more athletic teams like the 3 point shot does in basketball.