Whether you agree with it or not, there is a significant cultural stigma in the West around the word "cartoon". As others have pointed out, for a very long time it was almost exclusively associated with comic strips in the newspaper and then animated children's television.

It's similar to how "comic books" was set aside in favor of "graphic novels". "Watchmen" is not, colloquially, a comic book. Neither is "The Killing Joke".

"Animated film" is another example of a term that was adopted to distance some content from the stigma of "cartoons".

For a very, very long time in the west animated content was the exclusive territory of children's entertainment.

Literal, dictionary definitions may allow the use of "cartoon" to describe any animated content, but it's a lot like the word "sports". Its usage has evolved to imply a specific subset (athletic competition), even if you can technically say you're doing a variety of non-athletic activities "for sport".

And I say all this as someone in their 40s who truly enjoys American cartoons, adult animated series, anime, and a variety of other media generally skewed towards a younger audience.

So yeah, I don't call anime cartoons for the same reason I don't call hunting a sport :)