It's kinda built into the way Lemmy works - instances would crash if they had to drag in every community ever made. However, I think finding communities needs to be easier. At the moment, you need one of the external Lemmy search tools to find everything.
Not sure if this is any help but… I’m on the lemmy.world instance and just searched for audiobook communities on the Memmy app. Three came up including the lemmy.ml one, and all three let me subscribe.