An all-TTS audiobook offering is just about as appealing as an all-stable-diffusion picture gallery (that is, not at all).
There already are audiobooks on audible that are 100% TTS, while it's playable, it's no substitute (yet) for a real human.
It's just too flat/dead compared to a human reader.
Isn’t it more like an art gallery of prints of paintings? The primary art is the text of the book (like the painting in the gallery), TTS (and printing a copy) are just methods of making the art available.