A large part of it is the target audience.

Like western (or at least, American,) animation is mostly intended for children (Disney animation, Pixar. Paw patrol… looney toons,) or is of one of two genres (dc/marvel superhero’s, or like Family guy, South Park, simpsons.)

A lot of anime is intended for kids, too, don’t get me wrong. But a lot of it is also very much not. You also have a much broader array of genres, as well as a much broader distinctions in style in them.