Most DRM is trivially easy to fool: Just pirate. When you circumvent DRM, you are pirating, except you paid for the privilege. You probably also have a bigger risk of being caught, as you still run the DRM software.
well, you're still supporting the people behind the works, which - believe it or not - i like to do.
and also, while the iso/tv/movie scene has strict quality rules, the ebook "scene" seems more ad-hoc. pirated ebooks are sometimes OCR scans with tonnes of spelling mistakes or weird formatting, or ancient editions.
Also, there are other ways to implement copy protection. Eg Apple moved from DRM to fingerprinting downloads so torrented music could be traced to the leaker.
There could be other solutions that are harder to fake. E.g. a failsafe that an SSL pinned address returns 410 GONE. That’s stupidly cheap to run indefinitely.