Actors are only human, which includes voice actors. Voice actors don't have to worry about makeup or costumes, so it can be easy to assume that their job is much easier than that of actors visible on the screen. It may or may not be, but the unique challenge they face is breathing life into an animated character with their voice alone and no other props. And like with new artists, it can take novice voice actors some time to grow into their character and develop distinctive voices for them. This trope can also apply to singing, or any other medium that requires a voice in some way.