iOS literally doesn't have "just there for storage" directories that are not icloud. Either you use the photos pool, or you use icloud, or you use per-app directories that are local to each app. But then those directories cannot be accessed by other apps. Even if access to individual files is implemented, it's often very buggy.

Say you have a pictures directory shared via Möbius Sync. Then you want to edit one of the pictures with a pictures editing app that is not the builtin photos app. The app crashes, or cannot save an edited copy of the file into the same directory. I encountered this in real life.