It is not the monetary incentive. My friends who have contributed chapters to books generally see essentially zero return from royalties when the books are printed/sold.
Academics are more concerned about getting their name out (i.e. prestige). The business a professor runs is his/her name and they must market and position this well so that they get the funding and grants and the agencies recognize their name. There is nothing limiting professors currently from assembling notes into a textbook using free tools like LaTeX and putting their book on the web. Some currently do this (thank goodness!). However, putting a point on a CV that you uploaded a PDF file to your personal website versus being published by a well-known publishing firm makes a big difference.
That would be the problem to tackle: assuring authors that their time will be worthwhile. The fact that a book gets through the process to be published is seen as a mark or seal of quality, whether this is in any way true or not.