You seem to be operating in some fantasy world where Dez was just camped out in the end zone waiting for the ball and he accidently dropped it. It was extremely well-covered and that played a huge part in Dez not making the catch. He's made a lot of tough catches, but you've got to give some credit to the CB there. That was no gimmie play, certainly not as much of a gimmie as say.. letting Murray try to score from 3 yards out.
It is a play calling problem, because they decided that calling for the backup QB who hasn't thrown a meaningful NFL pass in nearly a year to come cold off the bench and immediately throw the ball twice inside the five-yard line, to a WR who is not used to catching balls from him, was a better idea than letting the best RB in the league, who just busted off 54 yards on his last two runs, try to get 3 yards on 3 carries.
What's the higher percentage play there? What logically gives us the best chance for 7 points?
That's a play calling problem. That's outsmarting yourself, and the proof is in the pudding.