I’ve been looking into it. Google published the design and software for the book scanners they used for Google books and it seems plausible to DIY them for a reasonable price. You can also buy book scanners for a couple of hundred dollars if you don’t mind turning pages manually.
The main thing that’s kept me for pursuing this is that I don’t really want to own all the physical books in the first place. I prefer physical copies of technical and math books, but for random fiction I am interested in ebooks to save space. Buying the books and scanning them gets me into the same position with books that I’m already in with movies, shows, and music: lots of physical media taking up space so that I could put a bunch of extra effort into getting a digital copy that I could have downloaded for free with far less effort if I were willing to pirate.