Why do they need celebrities to do Chipmonk voices?

They don't, really. But I think it helps to have a different person doing each chipmunk voice, rather than Ross doing all the males and Janice doing all the females. It often sounds a little bit forced/artificial when one person varies their voice to produce three different voices in the same film. (Watch Happy Feet, you'll surely notice Robin Williams' multiple roles.) The chipmunking only partially masks that.

I imagine they wanted reasonably young actors (not Tress Macneille or Rob Paulson) to more faithfully portray young characters, good actors to better play the characters (even though they have to do their voice-acting in slow motion), and popular actors for the box office draw.

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I wondered that too. I mean you can't tell who it is without having to look it up, so why not just hire nobodies to do the voices? It wouldn't have made any difference. Plus it would have saved the studio millions of dollars. Hell you can turn my voice into a chipmunk for a $1000! lol