Fellow German here, I think this actually makes sense.

Primarily, unlike the US we just lack the void space. Even if you're in a "rural" area you'll still annoy dozens of people firing a spud gun with compressed air, and one fired with explosives will be thought of as someone firing guns - which is damn rare to hear, only during hunting season in the forests. The same argument also holds valid for a number of other things common in the US but not in Germany like keeping entire residential properties filled with cars in various states of (dis)repair, firing guns in general, producing nuclear waste, flying experimental planes, starting planes and choppers from anywhere else than a licensed airstrip, ...