> The sillyness & waste of this is obvious

Until you realise that there is not one true way to show a map, and that different apps may actually have different needs. Suddenly it becomes obvious that not all apps can use the same shared offline data.

To further back this up, just because OSM might be a map's data source, it doesn't mean they use the same rendered vectors or images for the tiles.

I make changes to OSM so they can be propagated to a cycling-specific mapping tool I use (it's a commercial tool with their own custom map layers) - it takes about 3-4 weeks from when a change is made on OSM for it to be incorporated into their data set.

So yeah, it's not as simple as "we all use OSM so we'll just share all our rendered mapping values".