Because it's not a copy of the cartoon. That's why.
There's some of that as there is in every fandom, but the actual protests about this are not just about it not being like the cartoon, but about not being like the cartoon because they're following racist casting traditions.
People are going to enjoy this movie, "whitewashed" cast or not. People that have never even seen the show and won't know any better. Are they all racist? Are they all stupid?
Not sure where that question comes from. It's not about saying that anybody who sees the movie or enjoys it is racist or stupid (though I would think that depending on the extras and the set design people might indeed pick up on the whitewashing of the heroes). It's about questioning the studio's statements about race that they made through their casting. It's not like those people would presumably enjoy the movie any less if the actors were Asian.
Nobody blinked an eye when people said the Harry Potter movies should be cast with British actors. Casting Harry, Hermione or Ron with non-white actors was never in question. Yet it's not surprising that with Avatar the casting call was for Caucasian actors.
Does it suck that people can't just get a job no matter the race or color? Yeah. But it goes all ways. And even when movies or shows make a conscious effort to diversify a cast people still complain about the portrayal. No one will ever be happy.
Casting has changed as people have spoken out and made certain things unacceptable and other things more acceptable. Just because people--especially fans--will always have nitpicks about how their favorite things are adapted doesn't mean no criticisms are worth making. And questioning why white actors are always the default even when the characters are more associated with other races isn't any more pointless than defending the casting practices already in place. Yes, there's still going to be arguments and criticisms as people try to get better, but that doesn't make the effort not worth it imo.