Its not inexplicable. He doesn't know how to run or time routes. He is not precise. And the QB and the coach do not trust him.
His best two plays, other than drawing interference on Go routes, are hail mary's for which he was covered pretty well.
He breaks off his own routes and attempts to catch balls thrown to other receivers. That looks promising as a #6 WR or in the 4th quarter of a preseason game, but doesn't work as a stater.
Two years ago, during WR armageddon and great offensive drought, he ran a slant route from the slot and faced a ILB in his way. His attention was distracted by the Backer, he ran into him and got knocked on his ass for his trouble. He had ZERO idea how to adjust the route and zero idea how to draw contact in such a way as to get a flag called on the ILB.
I would use him only inside the 20 on specially designed plays because of his height and hops.