I'm going to fall back on my old staple of why there are no nude people in games.
America makes a lot of games, America hates nudity with a passion.
Not America as a whole, or even a majority, but America's 'Moral Majority' would throw a fit if a woman so much as expressed an interest in sex beyond that of pro-creatory processes. However, the Moral Majority are completely okay with blowing away thousands, as long as they are dirty foreign types who need to be returned to whatever Commie Hell they spawned from.
Think I'm exaggerating?
R Rating in America means Under 17s must be accompanied by parents, but in practice (so I'm told) means that a lot of people younger than 17 can go and see a movie or buy it without much hassle.
Kill Bill in America is rated R, that means that some pretty damn young people could see it if they so chose.
Kill Bill in England is rated 18, that means that no one under 18 can watch the movie legally.
However, if Kill Bill were to show even one solitary nipple in its entire run-time, I have no doubt that it would have been rated at M, or NC-17 or whatever you call it, in America.
Alright I'll stop being silly for a moment. Howevr my point is mainly correct. When the Briths or European boards of film and game classification look at a game they are mainly concerned with violent or mature content, and nakedness is not necessarily mature content. Sexuality is mature content, depending on the setting and use, but nudity is not necessarily. However, America's various classifcations are based more around the sexual content than the violence. Some extremely violent films, games and tv shows in America get relatively low ratings because they feature no nudity or sexual conduct, thus the best way to get a low rating in America is to just ignore the issue of nudity. However in Britain and Europe and indeed the rest of the world we care more about violence than we do about nudity.