It's not clear to me from this reply that you know very much about authors, advances, and the like. Maybe I'm wrong.

Anyway, writing a book is an actual job for some people, and not a hobby, a passion project, a speculative endeavor, a side gig, or a way to boost your leverage in compensation negotiations with potential employers. Seeing as how it's an actual job that entails a bunch of really hard work that I don't have to do and, frankly, don't particularly enjoy more than N number of other things I'm doing right now and that I'm good at, I'm not interested in doing it unless it can replace some subset of the paid work I'm currently doing that I enjoy (and learn from).