This is not an anti-Kindle rant. I have purchased (rented?) several Kindle titles myself.
However, YSK that you are only licensing access to the book from Amazon, you don’t own it like a physical book.
There have been cases where Amazon deletes a title from all devices. (Ironically, one version of “1984” was one such title).
https://www.nytimes.com/2009/07/18/technology/companies/18amazon.html
There have also been cases where a customer violated Amazon’s terms of service and lost access to all of their Kindle e-books. Amazon has all the power in this relationship. They can and do change the rules on us lowly peasants from time to time.
Here are the terms of use:
https://www.amazon.com/gp/help/customer/display.html?nodeId=201014950
Note, there are indeed ways to download your books and import them into something like Calibre (and remove the DRM from the books). If you do some web searches (and/or search YouTube) you can probably figure it out.
GoG, and physical games are only licenses as well. If you have any physical games from the era of instruction manuals you can find it laid out clearly inside, generally towards the end.
But GoG’s offline installers and physical games can’t be taken from you by the publisher etc (servers for online games and updates aside).
Neither can installed copies of games if you write protect the files, back them up where the launcher can’t get to them, etc. Licensing, DRM, and legality really aren’t the defining factors here. There are shades of better or worse, but at the end of the day it’s about simply being able to back up the media in a form that can’t be touched by the corporations.