My understanding is that Amazon runs imprints, like Audible Originals [1], that basically act like a publisher. I.e., they buy the rights, produce the audio, pay the voice actor, and the author gets some royalty (presumably smaller than 40%).
But my understanding is that the large publishers mostly do this themselves (and therefore have to pay the voice actor out of the 40% cut they get). Similarly, independent authors won't get access to this level of service; in some cases they're literally reading the book themselves, but they still only get the 40% cut (and that's with exclusivity).
The reason why audiobooks are cheaper is due to the subscription model. That's the chief innovation that Amazon pioneered with Audible. Now, their clout certainly also had something to do with it, and the burgeoning Kindle ecosystem. But the issue of price is really a matter of subscriptions being stickier and keeping customers in the ecosystem longer, making it possible to make aggregate more money. It's the same business model as Netflix, Dropbox, Adobe CC, etc.
[1]: https://www.audible.com/ep/audible-originals