It is simply because they haven't gotten to the point where it is necessary to have a reason to be embarrassed with your own nakedness.If we remove or change this conditioning (NAKED = UNCOMFORTABLE, BAD, etc.), you can bet things will be different.At first I was pretty grossed out by all the dicks, but now I don't really mind it.
When humans started to abolish divisions, when we want to re-discover our free consciousness and nature..anyway.. I think there is a decent chance that the taboo will disappear.
Things change all the time, and it isn't even one of the biggest taboos around anymore.
Religions, for millenia, have shamed their followers for being horrible, flawed beings. The human body is so taboo it's used for comedic effect on raunchy television because it's so rebellious to show it.
Most of these religions have become so pervasive in world cultures that merely showing a naked human body (you know, the one everyone has) triggers a double take. As an art student, I've had my share of life drawing classes.
Kjaggard, I know for a fact that America's general prudishness comes from our Puritan forebears.