It has to do with what people know, I think.  Up until V4, the jaws were attached to the head.  I still think that way myself.  So I look for the head controls on the head.

It's the way the real human head works. When you open your mouth, your lower jaw hinges down. Putting an ERC into the lower jaw to respond to certain morphs shouldn't be that hard, and would work realistically. It might have to do with reducing clicks.  Moving the jaw to match the mouth after the mouth is morphed IS more work.  That's my 2 cents on it.

Is this not logical?  If people want to move the jaws after wards, they still can.

I cannot save the world. Only my little piece of it. If we all act together, we can save the world.--Nelson Mandela
An  inconsistent hobgoblin is the fool of little minds
Taking "Just do it" to a whole new level!