Why do animated movies always pad out the cast with famous ppl?

I just got back from watching The Lorax at my friend's house and it got me asking myself this: Why is it that animated movies feel the need to pad the cast with a famous celebrity behind every character voice? Seriously, the kids don't care that a celeb is in the movie - they wanna see the animation - and much better voice actors could be hired who aren't famous. Why do Taylor Swift, Zac Efron, and Betty White need to do voices for the characters in the same movie? The voices didn't even fit the characters. And couldn't studios save a ton of money in making the movie by casting more talented voice actors who are less well known? They could save a whole lot because they wouldn't be able to demand as much dough as the big actors could. I know there are some celebs who do great voice acting work, but I really think celebs are stuffed in animated films way too much.

Anyone else agree?