This happens with Julio Jones all the time in a different way. The dude just does not get targeted enough in the redzone. Why?! He's huge! He's maybe the best receiver in the league! What's the deal?!
Football is beautiful because it is complex. The guy with the ball is only 1/22nd of the action on the field. Sammy might not be catching tons of balls, but that doesn't mean he isn't hugely important to this offense.
Why is there so much action open underneath and on intermediate throws? Play action is only a small part of the story, I know that much. The threat of Watkins is absolutely another part of the story.