What incentives do academics have to write text books?
Another reason to write an advanced text is that you have developed a body of research around a topic, and it's stabilized enough that you can present the material in a structured form. The incentive here is that the distilled understanding helps you understand your field, and it helps others work in the area as well. In addition, a well-written book can get you lots of citations, impact, and recognition (the same kinds of things you get with papers, but possibly even more).