3 per day is still pretty excessive? Who needs to publish 3 books per day?

Someone releasing their back catalogue of already written books on amazon for the first time.

Then there could be a special process for those cases, which are probably rather rare compared to what is essentially the ebook equivalent to spam (= essentially free to make, so with just volume you can make money even from inadvertent clicks or things people buy while barely paying attention).

There could and there should be a process for that. Amazon just doesn't want to spend the money for implementing this unless they are forced to do so.

Are we talking about the same things? How does a latency of a few days affect the author in that scenario? It's books not hft. Should be fine to limit much further than 3 per day.

Someone generating books using AI.

I imagine it's to allow for mistakes. You publish the wrong revision and you get a chance to undo and redo twice.