I'm not really into audiobooks, but my mom is, and she's lent/given me a couple. I think, for her, having a good voice actor is at least half of the experience, at least when she describes her favorite books half of her praise is for the actor.

Having listened to her favorites I can confirm the actors are really good. They are true professionals, far beyond what AI can do. AI can do commercial voiceovers, where there is purposely a single-note, unevocative tone. How can it do a shift in emotion across a line of dialogue as a character has a revelation? Or a slow change in personality as a character goes insane? Or slightly modify their voice as an angry, drunk father finally realizes he is pushing his daughter away, or his voice cracks when he knows the treatment is hopeless, or drops his guard when he remembers his old friend he didn't recognize? Etc. Even the pauses can communicate volumes.

This is the emotional landscape actors excel at navigating but tech bros aren't even aware of because of their terrible media literacy. So even if some "prompt engineer" was babysitting the AI it wouldn't be nearly as good. Basically, just saying the words is only half the actors skill, they are great at analysis also.

It's for making Jeff Bezos even more money