Mostly because certain apps refuse to adopt Android APIs, or insist NDK is a full blown GNU/Linux userspace, contrary to Android team official position on the matter.
The fact that the Android team's official position on API usage determines what software I get to install is exactly my problem with this gatekeeping.
The latest victim of this travesty is the removal of syncthing from the play store and the subsequent discontinuation of the app. This was ostensibly due to syncthing's failure to leverage the storage access framework to access files on Android devices. In reality, developers were benchmarking the storage access framework as somewhere around 50 times slower than direct system access, and that made it infeasible for usage in apps like Syncthing. That bug has been open for years, and the Android team has done nothing other than claim it's fixed when benchmarks show otherwise.
So I'm not sold at all on the value of these gatekeeping stores that have black box approval processes with changing rules. It is a system that is set up to be evil because it can reject and accept on a whim with no accountability. We should not so easily give up on installing the software of our choosing on the devices we purchase.
