Manga/Anime aren't genres. They're categories of cartoons and comics indeed, but not genres, as each manga/anime pertains their own genres.
I guess one reason of this unwillingness is the way that comics/cartoons are perceived in the west. Cartoons are generally perceived as a children's medium (despite the fact that the likes of South Park and Family Guy are still a thing), whilst comics generally just get blown off as superhero stories by the ignorant, forgetting that any kind of story can be told in a graphic novel. Overall, this leads to manga/anime being treated as superior by many due to it not having such stigma in Japan, despite many artists and series being inspired by western cartoons and comics, Osamu Tezuka and Akira Toriyama being prime examples.