Its important to remember that the very first animated feature was first shown, not to kids, but to adults. Walt Disney had some was rather apprehensive about showing an animated film to an adult audience, but look how it paid off for him. No, don't get me wrong, maybe Technicolour was more impressive in those days, but could you honestly see a film like that being shown for mature audiences now? It probably wouldn't even get a script.
I think the main problem is that traditional animation (in particular TV animation) has been drilled into a generation of people (specifically Generations X & Y) that he genre is for kids only. The lack of adult orientated content (Fritz the Cat aside) during that time period is surely a factor. When kids get older, of their is not a substitute for the shows/movies they are now too old for, then they find something else to fill the void left behind. And in the past, animation was certainly not the entertainment of choice.
Fast forward to today, and CGI is almost like an entirely new genre. Yes, it is animation, but to the average Joe Public, it looks completely different. Reasonably life-like faces, 3-D environments and sharp writing courtesy of the guys at PIXAR. Of course, these films are aimed squarely at the very generations mentioned above, why? They've got kids and disposable income and are quite happy to go see something that will keep the little ones happy. And if they can get a few laughs too, that's the icing on the cake.
Almost all of these types of CGI movies play to their strength in that they can get away with stuff that only a cartoon can (talking ants, cars, toys, animals, etc. etc.) Adult audiences looking for a more mature storyline aren't nearly as easily fooled. Unfortunately, the sort of films they are looking for are already very well catered for by live-action (as mentioned above).
I believe that it is quite unlikely that we will see a successful adult CGI movie in the foreseeable future. The most likely route chosen will be live-action using CGI to create things that can expand the expectations of the audience without distorting their view of what reality in a movie really is (The Matrix was perhaps the first modern one).
On the Japanese thing, that's an entirely different culture and it is well documented that animation has been a dominant force in the entertainment industry since the 60s. It has established itself as a mainstream genre with audiences, unlike in the US. Although this is cannot be used to solely justify the perception that Japan can create better adult animation.
I saw this at the weekend and to be honest, I'm not sure how it would have worked as a live-action film. For the amount of screen time that we actually see the characters in normal settings, the difference would be slight in all aspects. Besides, the character design was pretty good.
Quoted in addition to what I said above.