It complicates the Store UX, too, if they have to add "This book is/is not supported by your device" warnings to every book which also needs to know which device you are intending. With the average kindle owner often buying books directly from Amazon.com rather than the on-device Store and often having 2+ devices, they'd possibly need an exponential number of those warnings ("This book is supported by your Kindle Oasis and Kindle Paperwhite C, but not your Kindle Paperwhite B or Kindle Paperwhite A").

Also, maybe the publisher of that book in 2015 wants to upgrade to new ebook features for that book in 2026, for instance they want to add the physical book's original illustrations now that Kindle finally supports more illustrations. Does Amazon have to keep both of the 2015 and 2026 versions of the book depending on which device the user wants to use? How confused is the user when some of their devices have lovely illustrations and others don't? Should the user be able to choose to read the 2015 version of the file even on devices that support the 2026 version because they hate the book's illustrations and find them distracting?

(That gets into a larger discussion that Amazon has always preferred updating books in place on kindles with later editions as they are published, which archivists hate especially because the kindle doesn't have a great "edition version number" to rely on to track for when Amazon has delivered an update to a file, but which often consumers prefer because typos slowly disappear and books subtly become better than the last time you read them, presuming the Publisher isn't doing some drastic bait and switch and it focused only on "plussing" the book.)