There are differences between play store and other store. For instance, on fdroid you have a pop up asking each and every time if you want to install or update the app after pressing install button. Play store you just click install and let it do its work in the background.
I don’t think fdroid can update app by itself in the background. Play store can.
I mean third party store doesn’t have access to some of the api play store have, things you don’t care but that is important to normies and that would induce reduced profit from epic store.