Because the play calling is so bad. Looks like they just say alright Dez lineup wide right or left and do a go/post/hook or whatever route. If you somehow miraculously get wide open with that double coverage, we'll toss the ball your way. Otherwise, be a decoy and block for your fellow running back.
That's not how it should be. Have him go in motion, line up in different positions, in the slot, wide, or hell even in the backfield. Throw the defense off and cause some confusion. If you just line him up in the same position and have him run the same routes, he'll never consistently get open. Nobody would unless you have like 4.2 speed. Especially since Dez gets doubled almost every play. The fact that Dez still puts up great numbers with this senseless play calling tells you how great he is.