It gets back to my thought that all of this is by design. I'll expand on my original thought. Perhaps Mac is more Drew Bledsoe "nah, f**k that, go out there and sling it" and less Tom Brady take what the D gives until it's time to make a play. So Bill recognizes that Mac is too much gun slinger not enough game manager as a young player, so he brings in uber conservative fat Matt to counter that and not give Mac plays that let him give in to his tendencies. Bill knows he's not Mahomes, Herbert, Burrow or even Lawrence, so he has this infuriating conservative run his offense to limit Mac's propensity for slinging it all over and wait for him to mature until he recognizes when he can do that and when he needs to take the short stuff. Matt is just like Bill and he can't alter his approach, unlike JMcD who knew at the end of the half it was time to open it up. So we continue this conservative slog, irritating not only us as fans but Mac too.
It's just a theory that fits the facts right now. We will never know, but the more I think about it the more it makes sense. Bill doesn't want this kid to end up like Milfson with the Jete, so he's basically holding back the reigns and Matt is essentially the scapegoat, for lack of a better word. Bill knows Matt is going to err on the the side of caution, so he plugs him in to run it and not let the kid eff it up too badly.