Someones linked in this thread the Defunctland video essay on these characters that I highly recommend watching since it goes into this in detail.

But the main reason is, there's a lot of brand imagery on the line with these interactions, someone putting on a voice, or using a voice changer could make a mistake. Disney instead have a conversation tree with pre-recorded voice lines that a remote operator can control. Much harder to mess up

And possibly more importantly, much easier to keep doing for hours on end. There's no need for a highly trained actor.