Anime / Manga - Image #863,760

That's the case with any medium. A core audience is built upon with everything else being discarded. It's that way with video games, it's that way with TV and movies, I'm assuming it's that way to a lesser degree with books, and comics.

They're following the money. Sure, the studios are guilty for being samey, but the consumers are equally, if not more guilty for buying it and supporting the saminess.