I understand a lot of your points and the japanese mindset of 「もったいない」"mottainai" as in not to be wasteful. I think because of that, partially, that japanese end up forcing themselves to eat bigger portions here and also because they do not know of/are unaware of our "doggie bags" so to speak. In japan there isn't really this sort of system to get "a box to go" with your left overs. My girlfriend loves it, cos, being that she's here in America anyways and is getting served American sized portions, she can both not force herself but also bring whatever's left home to eat
So combine bigger portions + not to be wasteful mindset + not knowing about to-go and I think you'll find your answer.
As to the reasons why they don't have this in Japan, I could only speculate...