Why do people think Japanese cartoons aren't cartoons?

You have to consider the definition of what makes something a cartoon and what makes something an anime. Cartoons are distinguished by their simplistic, unrealistic, and sometimes exaggerated portrayals of things and include things from comics, books, and animation. Anime is honestly just a blanket term for animation in Japan, but it is stylistically distinguished popularly by having more realistic, more detailed portrayals of things even if they aren't 100% accurate with reality (exaggerated) and anime is exclusive to animation (similar to how manga and comics differ, manga are more comparable to graphic novels than anything). These distinctions make them two different entities instead of one

It isn't a mental disability if you don't research the definition of what makes a cartoon a cartoon. A person can call Cartoon Networks old "Cartoon Cartoons" Cartoons because their style reflects them, but the stylistically the same doesn't apply to shows like Legend of Korra or Young Justice so their just "animated series". The definition of a cartoon and anime has been debated and changed over the years. In contrast, while Crayon Shin-Chan is crudely drawn, its was Japanese made so it is classified as an anime television series.

Because they aren't. There are differences in the animation. You can tell the difference between a cartoon and an anime by the way they are animated...

Adventure Time, Regular Show, Simpsons, American Dad, etc, are all cartoons.

Where animes can be identified by the characters having distinct features, such as the large eyes, their hair, etc... some animes include Pokemon, Attack on Titan, Madoka Magica, Death Note, Steins Gate, Kokoro Connect, etc...

If you watched them, you would be able to tell the difference.

It comes from comparing them to western cartoons. Western shows are generally aimed more at children, they're fun, colorful and weird. Only a small number are tweaked towards older audiences.

In contrast, most anime is tuned to teenagers and adults. Anime often deals with much more serious topics and, depending on the show, is usually not meant for children. When you call anime "cartoons" it's often taken as an insult that you're calling it a kid's show, even if it isn't. So even though technically they are both animated shows, fans often consider anime and cartoons to be completely separate things.

Japanese cartoons are so amazing ,perfect, different and real that they deserve a new category^.^

Which mental disability do you have to not know there's a difference?