I agree, the issue is that the map data is highly customized. I believe StreetComplete uses online map tiles so that's less of a concern, but i.e. with Organic Maps the map data is highly tied to the app version: support for a data entry needs both app/rendering/logic support and presence in the data structure, and full forwards/backwards compatibility isn't always possible. The map files also need to be optimized for Organic Maps' speed/usability improvements over apps like OsmAnd: pre-indexing, etc. Maybe someday there's one standard format for it, but for now each app makes its own map files.

Also, mobile apps often have strict privacy lately around what files they can access: they're not just sitting on the filesystem, they're in access-controlled app-specific folders. That's good for privacy/security, but a dealbreaker for first-class sharing of information between apps.