Pretty simple actually. It all comes down to audience.
In America, Cartoons are automatically seen as "kiddy stuff", or "Comedy". That's what people expect, that's what people get. (Spongebob, Simpsons, etc.). In Japan, Cartoons (or anime) aren't seen in such a restricted manner. As a result, Anime can explore more mature themes and cater to an adult audience.
It is simply that the culture for making mature movies / shows prefers to make live-action stuff in America. I mean, we've got Lost, Heroes, Prison Break, etc. etc. Its not that scriptwriters can't make mature storylines... its just that it is a cultural taboo to make a mature
cartoon. No one in America would make a cartoon about a Meth Dealer... but they are willing to make "Breaking Bad". Even comic book art aims for photo-realism when the story gets serious (DC's "
Kingdom Come" for example). Well... not so much anymore. But mature comic books seem to borrow their styles from Anime actually. (or maybe I got it backwards? I am not a historian...)
Anime is not special either. Even in Anime, various styles give connotations to the maturity of the plotline. More Mature animes such as Death Note and Evangeleon tend to stick closer to human proportions, while One Piece and Pokemon are willing to be a little more cartoonish.
There are exceptions of course. But for the most part, in America... Live Action / Photorealism implies maturity. While Cartoons imply Sophomorism. In Japan / Anime, it exists to a much lesser degree, but you can still see it.
Fortunately, Anime is breaking down that boarder. Unfortunately... the future of Cartoon Network seems to be in "Chowder", "6teen" and other crap shows that will only secure cartoon's connotation of childish and/or potty humor.
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Anyone here an animator? There might be a chance that Anime is actually more expensive than we might think. I know for a fact that 3d models are less costly to make than a full 2d Sprites Sheet... which is why video games have moved towards 3d. Even BlazBlue sprites were rendered in 3d before they were handcrafted into 2d sprites. In the gaming world, 3d is cheaper. Is that the same for Anime?