Because of money and infinitely specific contextual garbage created by small art communities that help shit-scribbles become elevated to exhibits. Duchamp's toilet seat was proof that anything can be accepted as art if it's created by a renown artist.
In the long run, it wasn't a good idea.
Meanwhile, most cartoon-fans expect some form of 'tangible' artistic/writing talent. However, cartoon fans themselves mainly use cartoons for escapism and don't bother with becoming articulate with artistic theory/lit theory/etc... They simply hop into whatever comic or cartoon world they like and then whine when their rising expectations aren't constantly met.