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The issue with encryption is that the reader app has to be able to decrypt. So you have to use a standard scheme. Since one of the revenue streams for DRM owners is adding DRM for publishers, they don't tend to be interested in inexpensive encryption options.
In principle, the Adobe password-based ePub encryption (most used by Barnes and Noble, but supported on several reading devices and apps) could be reverse engineered and made available to everyone. No one has done this because a) those with the skills needed are more interested in DRM-stripping than DRM-adding, and b) Adobe might sue the authors of the encryption program. I don't know that Adobe would have a case, but that never seems to stop large corporations.
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