Or the people who say that are obsessed with subtlety and want everything to conform to the Western standards for art due to cultural hegemony brainworms? Sometimes I feel very guilty for engaging with this sort of media instead of something more "serious"
A lot of it is based on assumptions, stereotypes and worst of examples of the medium - generalizing it all
"1000 year old dragons", Isekai, squealing voice actresses, incest, mindless fighting for no reason, catgirls, giant robots, hentai, etc.
Stuff that is not absent from Anime (the reputation had to come from somewhere, I suppose), and people instead just prefer to follow no investigation, but I'll speak anyway.
Add to that bad reputation, some racist presuppositions against Japanese people, and for leftists "they did WW2, they're more morally bad and their art is to be mocked". Also, the main demographic of Anime and Manga are young people, teenagers. Shounen and Shojo for example; very common.
It is of course possible to create meaningful, mature art within the medium, even within the confines of works that usually wouldn't count. Oshii's Patlabor 2 or Urusei Yatsura 2 for example.
I love this anime but damn, if I didn't know it was made by a woman I'd assume it was made by a creepy guy.