This issue isn't specific to the Kindle, you can go and buy books on Apple or Google or Kobo or Barnes and Noble and its the same story.

Many books can only be bought this way. Publishers don't want to sell books through platforms that don't have DRM, because they are scared of piracy.

It would be great if we could have open ebook reader software that could display these books.

eReaders like the Xteink X4 are beautiful but lots of books are impossible to legally read on it. Yes, I know that I can break the DRM, but I think its sad that this is the only path for legitimate purchases.