I think the biggest reason that big city schools don’t dominate college football is simply because they aren’t the flagship university in the state. Most big time P5 schools are land grant universities, and probably have the deepest ties to the state and the people in the state. There hasn’t been a single city school relevant nationally since the Pete Carroll USC days. Maybe Washington recently, but nothing like what USC, Miami, and Texas have done in the past.
Speaking of Texas, although Austin is a massive city, UT is still the state’s flagship university and was probably a land grant school during its establishment. Same way with Ohio State and Columbus. Just the way cities developed overtime, college athletics kind of became irrelevant, and fans in the area became more interested in pro sports. Vandy baseball is really the only city team in any of the Big 3 sports that’s consistently competing on the national level. Maybe UCLA basketball in the past, but not recently.