This is a historical decision which we are open to revisiting. My guess is that this was done to avoid conflating the WordPress author of a given chapter (which has no impact on outputs) with the author supplied in chapter meta, which does, and also to avoid less technical users being confused by having an unordered view of chapters and front/back matter which does not reflect the book’s actual structure.
Revisiting the organize page is a key UI + UX goal of our revised development roadmap (coming soon!), so this is a good point to include in that spec process.