It's a common misconception that the pitching in AA is a higher calibre than AAA. Yes, the very top pitching prospect for a team maybe in AA, but that's usually 1 guy out of entire rotation, with the rest filled out by journeyman minor leaguers who may not even have sniffed AAA. The average age of pitchers in AA last season was over 24 in all 3 AA leagues.
Yes, AAA is typically older guys who once we're in the majors and are trying to hang on and get another shot, bUT they could still pitch better than AA rotations as a whole.