It's because it is about the voice. A good voice actor can produce many voices. Yes you can find someone else but it almost always is not identical. And since you have to hire someone anyway and they are good, what even make them pretend to be someone else? They might have better voices.
With computers you can essentially steal someone's voice and the way they act and use that for cheap.
Those companies cry about piracy, that someone downloaded something and that made them not purchase the movie, music or game that many wouldn't purchase anyway.
This actually is the real piracy. They can steal the likeness of those people and make profit without needing to hire them ever again.