No offense, but all the breeds you listed were bred for extremeties of some sort, not moderation. That's like asking why APBTs aren't huge like Great Danes or tiny like Chihuahuas. The breed calls for moderation, because extreme features detract from their ability to do any kind of serious work. The APBT is a dog that can do anything and excel at most of it. He's the ultimate versatile athlete because of his moderation. That's why when you talk about heavier skulls and such, I can't help but shake my head. A heavier, more massive skull would simply be more weight a dog would have to drag around, which would be a hinderance in anything. That doesn't mean some people don't breed for that, but it doesn't make any sense to those who use their dogs for more than to keep the couch from flying around the room.
I know you aren't trying to claim I have an affection for the word "gameness." I use it very rarely myself. It was last in the list of 5 different options I mentioned. How many terrier people in general call their dogs game? Or plucky, brave, gritty, stubborn, courageous, tenacious, etc etc etc. Dog people put a value on a temperament like that, no matter what name they call it by.