PGs in particular can just bring the ball up and shoot. (Not every time, of course, they're involved in set plays run through the offense, etc.) Once you start running plays for big men you introduce more opportunities for turnovers through the extra passing and dribbling to get the ball to them. You just get more turnovers running plays for big men than running plays for guards, and I think that's part of it.
That said, it does seem harder than it should be to get big men their shots at times.