While book scanning can be done by machine, the machinery is going to be expensive and complicated. Your typical bibliophile can't afford it. Scanning a book by hand can take hours, even with a V-shaped book-scanning fixture and two cameras.
The technology for digitizing audio is much easier to acquire and use. Any audiophile can afford the hardware and software to do a tolerable audio rip. Anyone can set up a rip, or several rips, and do real work while the rip takes place in the background. The quality might not be to audiophile standards, but will satisfy most casual users.
Even after you've created an "ebook" of page images, it isn't really suited for use in modern ereaders. For that you need an ePub format, or something similar. The text has to literally be in text format to allow reformatting. A decent modern ebook can adapt the text to different display sizes and different type sizes. This is hard to do.
Compare a typical book produced by Project Gutenberg and a typical book scanned into the Internet Archive. Gutenberg produces true ePubs consisting of text possibly sprinkled with digitized illustrations. Gutenberg might start with automatic text recognition, but its books go through a distributed proofreading process before they're released.
While I value what the Archive does (any digitization is better than none at all) I've discarded most ePubs I've downloaded from them. There are simply too many typos in the text recognition. Their scanned raw images and PDFs are usable, though they lack the flexibility of true ebooks.