Short answer: someone searching on Google is often still researching; someone searching on Amazon already has their card ready. For a book sold on Amazon, that intent gap makes Amazon Ads convert better per dollar spent.
A Google search on a topic can end in a purchase, in reading a free blog post, or in nothing. A search inside Amazon almost always ends in a buying decision, between your book and direct competitors on that same results page.
If you have your own sales page outside Amazon (say, selling a PDF or a course directly), Google Ads regains its purpose, because you're competing for traffic toward your own site, not within Amazon's ecosystem.