The cost of the voice actors and the additional coding required to do this would probably make this unprofitable.
You would have to include all the ghost comments from previous content and keep the voice actors on contract for all future content
You would not be able to change these for someone else (like they did with Dinklebot) if the actor was unpopular, unavailable or increased their fee because people paid specifically for these voices rather than being a small part of an overall product
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You could argue they are purchasing the ghost as is, with no expectation that future content will ever be recorded for it. Possibly they could even charge per DLC. And that's where it starts getting complicated. Of course if they did that then you should be able to set what voice you have depending on what piece of content you are playing.