We haven't gone back, because of who runs human culture. We most certainly do have the ability. Since the 2nd world war, the amount of design skills, materials, titanium, steel, precious metals all kinds of stuff and all kinds of motivations to efficiently kill other human beings. Had have been employed in the task of terraforming the moon, instead of the minor killing sprees we keep indulging in, I have little doubt there would be millions of people living up there by now. All you have to do, is ask who runs the Media?