Also, developing your own OS is not necessarily cheaper than porting Android. Boox has been doing Android-based e-readers for a long time and their devices are usually on the cheaper end for the hardware. I feel it boils down more to UX rather than cost - do you want to let the user install whatever apps they want, or do you want to give them a more curated experience (and probably also limit them to your walled garden).
I know you can de-DRM Kindle books, but there are other walled gardens that don't allow you to do that easily (for me it's some Chinese e-book apps).
My go-to e-reader for years has been an ad-supported kindle that I immediately jailbreak to install KOReader and remove the ads (as well as any communication with amazon). It's cheap, supports epub, and the hardware is surprisingly good and durable.
As a bonus you can basically install any GTK app you care to on there; I have a term emulator with SSH and a chess game with AI on mine, for example
The thing that nobody realizes until they actually spend some real time with eating is how much more comfortable it is to read with then a phone or iPad screen. It works in all lighting conditions, the battery life lasts for weeks. Good luck reading on your phone at the beach on a sunny day.
The things that it's not good at are non-text based games and videos, but that's not what it's for. I have a computer and phone for those things.
In addition to that, lending ebooks from the public library provides (at least some) income to the e-book authors and editors.
The library also has the current (clickbait-free) print versions of newspapers as e-papers, which the shadow libraries (at least those that I know) have not.
They synergize very well, though. :)
Performance is always in tension with price and weight, but software openness isn't in tension with any of them, really.
Yeah if you discount the one huge and obvious advantage of this approach it’s not clear why anyone would do it.