F droid is known to be highly insecure. It has many many bad practises that totally compromise security and they have proven to be woefully incompetent and ignorant to concepts as basic as the purpose of app signing.

The all apps stemming from app stores in the builting App Store is to provide a minimalist experience by default whilst keeping google play apps accessible. GrapheneOS has a majour focus on accessibility. They avoid users having to be technical to install an app store to get their apps.