The decodable and leveled settings are collection-wide, but to set them you have to create a book and open the relevant tool.
What if…the settings were configured in the Collection Settings? You’re doing the same thing, but you’re doing it from the top level, not when inside a book.
My rationale is that, just as with formatting in Reader Templates, you don’t want your average user fiddling around with the decodable and leveled settings. If you do it in the collection settings, people who are just using the settings while writing are much less likely to mess with them, and it’s also possible to hide the collection settings from the user, just like the formatting controls are unavailable if you are writing using a reader template.