There are a few fundamental reasons: 1. at least iOs app privacy/sandboxing model makes sharing any data files pretty difficult, if impossible. I suspect Android has similar issue. 2. there is no such thing as "just map data" - every app has to do opinionated decision what they need in their maps data (e.g. aerials, elevation, choice of POIs, languages etc), how it gets styled (it makes requirements to actual data). And this is for just simple base map data, you have also routing, address search databases etc on top. So there is not much to share really, and not easy to share. Not even talking about huge challenge to standardize it. (I'm founder of Nutiteq, we also did FOSS SDKs for offline maps)