Because knowledge is power and most people don’t like giving whomever power over them for no reason. Also, it shouldn’t matter why privacy is important to people, the fact that it is should suffice to protect it.

I’ve been using the pro version, Moon+ Reader Pro, for years. It’s great for reading EPUBs, which I either buy DRM-free or, if that’s not possible, in any format and then download a “liberated” copy from Anna’s Archive.