Some time ago The New Yorker published an article on why animals are used in stories instead of humans.
To quote from the article
Writers aiming to tell us about human life have often done so under cover of telling us about animals. Animals are fun—they have feathers and fangs, they live in trees and holes—and they seem to us simpler than we are, so that, by using them, we can make our points cleaner and faster. With Madame Bovary, you pretty much have to say who her parents were. With SpongeBob, you don’t, and this keeps the story moving. Most important, the use of animals to stand in for human beings creates a fertile ambiguity.