There's really no logical reason that famous film stars are also billed prominently for animation, and yet that's what we have.
The vocal performance and personality of the actor shapes and defines the animation of the character.
Disney understood that from the beginning, which is why three generations of stars from film, radio, television and theater have recorded for Disney. Try imaging the animated Aladdin without the manic improvisation of Robin Williams.
For bonus points, try re-casting the voice of Rocket Raccoon and see if you if you still have a CGI and motion capture character that audiences will actually give a damn about, help anchor a new franchise and deliver a billion-dollar pay-off at the box office.