That gives you 3 weeks of playoff games, enough time to set up bowls, and the participants get gate money by having big playoff games.
It also forces big schools to play better teams in the regular season, as scheduling Buffalo becomes less feasible with only 1 or 2 extra games to use, with a need to impress. You'll hear the small "I wanna be DIA" schools complain, because they won't get their "bludgen us to death so we can build a field house" money, but who cares. Some of those schools shouldn't be DIA or anywhere near it anyway.
It's not hard. It's just that nobody wants to do it. I can hear cash registers ringing with a playoff system that keeps the bowls in tact. And it's not something that would be incredibly hard to do. I find it hard to believe that University presidents; our bastions of higher education; can't come up with a decent plan that would also feed the coffers.