It is a cycle that perpetuates itself. Think of this
You're 17 years old and a top recruit. You're one the fence throughout the fall, but suddenly bowl season hits and 3 of the schools you are interested in get non-stop ads for an entire month on ESPN. Alabama this, Clemson that, Ohio State that. Talking about their tradition, their facilities, their coaches. They talk about it on Monday Night Football. That could be you. Playoff games, baby. then you watch the games, you watch the national championship. Its signing day, and by now you've had basically two months of big network indoctrination towards 3 schools. Why on earth would you go elsewhere?
and that is not the only part of it. We all know college football is broken at many levels, but the 4 team playoff and ESPN are further driving it into the hole.