Animated films (anime) and graphic novels (manga) are enjoyed by people of all ages in Japan, as one example. In the US we think of them as cartoons and comic books--"kiddie" stuff. Another example I'd use is the movie ratings system. If someone were to tell you they just came from seeing a rated "G" movie, you'd wonder if they had taken their kid to see the latest Disney film or something, when in reality "G" is for general audiences and is intended to categorize the types of movies everyone can enjoy.
But here in the good 'ole US of A, if there is no sex and violence, we leave it for the kids hehe.
Nintendo's creative endeavors have always catered to the Japanese with the hope the North American and European consumers will find the games interesting as well--it will never happen the other way around.
As for third parties--the "adult" games tend to be simulation games rather than amusement games, and those generally work better on the graphics oriented systems.