How about "we watch anime because it's generally better drawn, the voice actors have talent, and the topics are worthwhile?
Spongebob - I won't go there. The art is so lousy, it hurts. Most American animated shows are poorly drawn and badly voiced. While some in the last fifteen years have been funny, like "Animaniacs" or "Pinky and the Brain", very few have been meaningful in any way.
Anyway, why should I watch Cartoon Network when I can download the latest show from Japan? Anyone who likes "Fairly Oddparents" should see "Full Moon wo Sagashite", a nice little Japanese story with a slightly similar storyline (child has a pair of magical beings granting wishes) only "Full Moon" actually does more than slapstick with the premise.
Anime is about the only place to see decent science fiction on the tube these days.
That, and ninjas.