Pretty tough to administer a rule like that, and there would be a lot of grey areas. Is a sneak on third down only, from the 1,2 , 5 or longer, what about a fake handoff and the QB runs off tackle. What about a running back who comes and takes the snap on third and short and scores(has happened). What if the punt holder is a QB and picks up the hold and runs it in. There would need to be so many rules and the administrator would have a heck of a time trying to figure each scenario and there would probably be a ton of arguments. Best to keep it simple, there are many times a third down QB gets shut out in a game.