Frankly, I think the reason people haven’t built apps on top of Wikidata is that the data there isn’t very useful.
I say this not to diss Wikimedia, as the Wikipedia project itself is great and an amazing tool and resource. But Wikidata is simply not there.
But I tried to do some Wikidata queries for stuff like: what are all the neighborhoods and districts of Hong Kong, all the counties in Taiwan, and it's piecemeal coverage, tags different from one entity to another, not everything in a group is linked to OSM. It's not a lot of improvement over Wikipedia's Category pages.
A big part of that is that different language editions of wikipedia are very decoupled. One of the goals of wikidata was to share data between different language wikipedias. It needed to be decoupled so it was equal to all the different languages.
The OpenStreetMaps model is also interesting. Where they basically only provide the data and expect others to make Apps/Websites
That said, it's also interesting that there hasn't been any big hit with people building new apps on top of Wikidata (I guess the website and Android app are technically different views on the same thing)