If an author does not intend to make much revenue from a book, why not make it open-access? Many authors do not intend to make much revenue from books (textbooks or research books) they contribute to, yet they don't make it open access. Why?
There are a number of reasons why not to, and they stem from the reasons one might want to publish a book, even if you aren't making much if any money:


The prestige of the publisher matters. For many tenure committees, professional organizations, etc. "A Book from BigDeal University Press" > "Some Markdown Files on Github" or what have you in terms of evaluation. In effect, you are getting paid, but in prestige and reputation rather than money.
Publishers take care of a number of things that, if you're self-publishing an open access book you have to do yourself, including copy-editing, layout, and most importantly, finding peer reviewers.
Seeing a book adopted widely (another part of the whole prestige aspect) will likely be more difficult for an open-access book, at least at present, where they are fairly common. Who is going to do the marketing? Has it actually been properly peer reviewed? Are there nice, hard-bound copies available (some of us like reading things on paper), etc.


That is not to say that there are not some very successful efforts in my field to do open access books. Hernan and Robins causal inference book, for example, has drafts and code available online: http://www.hsph.harvard.edu/miguel-hernan/causal-inference-book/

But neither one of those authors is in the position to need much benefit from a book publication, and it's still being placed in a traditional press when it's finished.