If you recall the movie, Monsters borrows concepts that are Hollywood-like. It has sets that are like movie sets where the monsters are trained to scare kids.The difference is, Cars (while not technically set in California) has enough to do with California culture and a similar topography to feel Californian and thus contribute to the theme of the park. Monstropolis has nothing that is inherently Californian about it, specifically nothing that contributes to the theme of Hollywood circa 1930.
On the same token, Avatar is not about the celebration of man's realtionship with animals, both of real life and of legend; instead it's about a race of humanoid aliens who have no basis in real life or myth. In both cases these additions are operating on backwards logic, they're not saying "okay, here's the theme of this area, what can we do to contribute to that theme?" instead they are saying "how can we bend the theme to justify the inclusion of these popular franchises in this space?"
As I said, backwards logic.
Animal Kingdom always left the door open for "fantasy" concepts of animals. The dragons of Avatar fits. It even has a mother tree like Animal Kingdom. There is no backwards logic.
Besides, the myth is already taken with Everest and that disco ball. A myth of a myth.