If you were to add another game each week...people would skip games because there are so many or because there's another game. When there is only 1 game a week...it is scheduled, reserved, and prioritized above everything and becomes a must-see event!! It also makes it CRITICAL to see each game because one game is 1/12 of the season and has huge implications. It also allows for fans to completely comb over ever scrap of detail during the week from over-analyzing the last game to doing the same on the next game. It really allows a fan to go all-in on football.
You look at baseball and basketball and there are a lot more games but people tend to not pay attention as much because they can skip one no big deal. And one game is 1/162 or 1/82 and so not as much impact on the overall season. It doesn't allow the fan to be all in because the next game is tomorrow.