It’s because the first few generations of DRM were extremely poorly implemented. My og kindle keyboard still works and will ignore the DRM (that would be locking me out of, for example, a library book after its due date) if you just change the file extension to one of the DRM free file types. It will also then let me distribute that ebook to others without restriction.

It’s been a while since I’ve heard about libgen and aa - and actually i’m not sure how they operate with direct downloads of copyrighted material? I find my ebooks through more conventional p2p means, but i’ve always just assumed that was necessary to avoid sudden takedowns