For some reason, celebrities do a lot of voice-acting in big-budget animated movies. Maybe the producers figure more people will see movies with "big-name stars" in them. Maybe John Q. Public just loves his celebrities. Regardless, regular voice actors often get cast aside in favor of someone more famous when a big, important animated movie is made for public consumption. Often turn out to be examples of Ink Suit Actors. See also Pop Star Composer for the musical equivalent.