You do know that APKs are just any app, right? Even if downloaded from Google Play... It's just the installation tunnel that changes, and this is where a Dev can decide if they go with better odds through a mainstream, more restricted channel, or if they go 'indie' and seek revenue elsewhere, with the drawback of not being the official store. Or they could not be in it for the money and just release the app file for everyone to install (or perhaps because it does something that doesn't bode well with Android guidelines and cannot be anywhere else but out in the open for rooted users to get).