He has.. once he gets this 2nd movie out, he'll be everywhere.
Maybe Cena is questioning Rock's work ethic? I'm sure you all know Cena was out on the road for 2 weeks at a time when he was plugging the album, doing press for WWE, and doing house shows/Raw. Maybe Cena feels like if he can push himself to do all the extra stuff outside the ring and still show up for every show, Rock could've done the same thing.