Probably because Plex doesn’t have an Audiobook Library (Yet, we can dream).
Your Audiobooks are just “spoken word” music in the (aggravating) world of Plex.
You created and “Audio” Library that Plex just see’s as a Second “Music” Library, even if you go through the trouble of setting up the Audiobook Metadata Agent (Audible) (since abandoned I think) it’s just music to Plex.
But I highly suggest you hop on over to Support for Audiobooks Feature Request and please add your vote!
