It's true that the .esi file is just a zip, and it's true that the PDFs and EPUBs are all there. However, the meta data, like the db's and annotation files, may still be somehow encrypted using the original device's ID, or the issue may be the Adobe registration for the original device will not be valid on the new device (since each device registers uniquely with Adobe).

They're the ones that created the backup/restore system for the Library, so it was in their control to enable/disable restores to other devices. I'm sure they must have had a good reason, either security or Adobe.