Celebrity Voice Actors

Following through with what Cheese said, why does Disney have such luck with celebrity voice actors? Disney, primarily Pixar, casts a lot of celebrities, from Woody Allen and Tom Hanks in Toy Story, to Ellen Degeneris in Finding Nemo, to any number of celebrities not known for their voice acting. And yet, their movies are typically phenomenally well-acted, usually because of the life that these celebrities bring to the characters. And yet, when Epic uses celebrities, they show their lack of expertise in an admittedly difficult field.