People don't buy Kindles for the hardware. They buy Kindles because they work with a store that has the books they want to buy (and the hardware is good enough and cheap enough).
Without data I believe that more than 99% of ereader users have no idea what a MOBI or ePub file is and have never attempted to manually move a file onto their device. Instead, they go to a digital book store, click "buy", and content appears on their device.
Amazon does that as well as (better than?) any competitor, and crucially their store has the books people want.
I don't want a subscription store filled exclusively with LLM-generated trash. I don't really want to have to find which of a thousand websites will sell a DRM-free copy of something so I can buy it, download it, transcode it in Calibre into a format my device reads, and transfer it to my device. I'd much rather pay a few dollars, click a button that sends some money to the author and sends a book to my device.
This whole comments section reeks of "I don't understand why anyone uses Windows when Linux is so easy to get to", managing to simultaneously assume every user has a degree in computer science and overlooking all the rough edges.