I guess Valve would half-qualify as AAA? The top played true AAA game would be, imho "Grand Theft Auto V Enhanced", at the 18th place. Perhaps Ubisoft with a Assasin's creed title in the top sellers probably should be higher in the playing list.
But I think it's fair to say AAA doesn't hold the steam store anywhere near as tightly as it holds the consoles.
I think this speaks more to the type of games people want in their pocket. Games you can casually pick up, put down.