I wouldn't mind going even further - go to 22 conference games for a true round robin.
Coaches/ADs would be afraid the extra losses would hurt them in selection. But (1) selection relies less and less on W/L record and more and more on other things, and (2) if all P5s play more conference games, there are fewer quality games for mid-majors. The P5s can hurt the mid-major SOS while boosting their own.