The whole setup makes installing apps from other sources obtuse and 'scary' to the average user.
I'm not sure how say an app from f-droid is more 'unknown' than an app from the play store myself. Then of course, just to install f-droid, I have to download it, allow installing apps from my file manager, which comes with some scary popups, then I have to again enable the setting for f-droid, again after some scary popups.
The goal really does seem to be do make the idea of installing software one acquires outside the 'official garden of known apps' to be something terrible and scary nobody should ever want to do.