I noticed on github that some developers offer to download the app from Google Play without offering other options to download the compiled file ( the compiled file is missing on github too ).
This does not only apply to applications that are in beta. Why is that?
Cost. Not financial, time. I have to release on Google Play, because everyone is there, so its the easiest place to attract users. Adding fdroid or whatever else doubles the amount of time and effort i have to spend on releasing my app. Even creating a release on github isnt free in terms of time and effort.
Also why bother releasing a plain old APK when someone can use apkmirror to grab it manually.
And if you are releasing the app manually you need some handling for updates (or informing users to install an update on their own). At least in Google Play users tend to stay relatively up to date.