Why Do Animated Films Use Such Famous Voices?

I took my four children to the movie Coraline this weekend. After the movie, I asked them how they liked it. Their four answers: “great,” “good,” “O.K.,” and “Thank God it is over.”
Coming from my kids, who always say the latest movie is their favorite, those are not very positive reviews.
I have never been in a movie theater full of kids as quiet as it was at Coraline. That quiet, along with the plodding pace of the movie, left plenty of time to ponder things.
First, I couldn’t get over the fact that the name of one of the children in the movie was Whyborn, known as Whybee for short, as in “Why be born?” Whybee didn’t seem to have any parents, although he did have a grandma who would yell for him from time to time. It made me think of the unwanted children/abortion argument in Freakonomics.
Second, two of the voices in this animated film were done by Dakota Fanning and Teri Hatcher. The last movie I saw was Bolt, with voices by Miley Cyrus and John Travolta. The list of stars who have lent their voices to animated movies goes on and on: Eddie Murphy, Dustin Hoffman, Cameron Diaz, John Goodman, etc.
Why do big-name stars so dominate the voices in animated films?
One hypothesis is that they are better than other people at doing the voices. I’m almost certain that is not correct. I have to believe that there are a group of voice actors and books-on-tape readers who don’t have the faces to be movie stars but have great voices.
A second hypothesis is that the big stars don’t charge much for their voices. Doing the voice for an animated film doesn’t take much time or effort, at least according to this New York Times article. If that is the case, then maybe the cost of the actors’ voices is just a small part of the total cost of the movie; but I don’t think that is the case, at least not always. One source online reported that Cameron Diaz and Mike Myers each got paid $10 million for their parts in Shrek 2.
A third explanation is that people really like to hear the voices of the stars. I tend to doubt that story as well. With a few notable exceptions, my guess is that audiences couldn’t even identify the voices of the stars if they didn’t see the credits.
A fourth hypothesis is one that sounds odd, but will be familiar to economists. Under this hypothesis, it isn’t that famous actors are better at doing voices, or even that moviegoers like to hear their voices, or that stars are cheap. Rather, big-time actors are hired to read these parts precisely because they are expensive.
In order to be willing and able to give multi-million dollar deals to stars to do voices that a no-name could do for $50,000, a producer must be confident that the movie will be a big hit. Thus, the big star is hired solely to give a credible signal to outsiders that the producer thinks the movie will be a blockbuster.
Ultimately, I’m not sure any of these hypotheses really feel right to me.
Any ideas?