We have a playoff to emphasize it.
In the past it was not that big a deal that there were only 5 or 6 elite football programs. Sure, we always know there are only a handful of teams that can win a National Championship at the beginning of each year. From 1989-1993, ND was one of that handful. We got a little fortunate in 2012 and 2018, too, but probably not a real threat. In the past, this didn't matter so much because the other 110 teams were playing for prestigious bowls instead, and playing for position on the top 25 ranking. We cared about No. 1, but the teams playing for no. 1 just played a bowl game like everyone else.
With the playoff, there is so much more emphasis on the playoff itself that no one cares what happens to the other 118 teams. Positioning in the top 25 just doesn't seem to be important anymore. Expanding the playoff is not the answer to that, since you are just polishing the same turd.