His return game isn't the problem, but his ROS itself is.
Fed and Djokovic are so good at putting a neutral rally ball away that Rafa's preference of hitting neutralising returns from so far back behind the baseline (regardless of how hard he hits it) plays right into their hands.
Putting more pressure on their serve game right from the return will put more pressure on the server to hit better serves, which introduces more second serves to capitalise on.
But when you pose no danger to their serve games yet allow them to get into yours, then suddenly they can afford to take risks in your serve games.