https://guardianproject.info/fdroid/
Basically, if the app developers bother to maintain their own repository, there is no reason for other f-droid repositories to waste storage and bandwidth on duplicating them and constantly checking to be sure its the latest versions of those apps that have been copied. This is a feature, not a bug, in systems like f-droid.
That said, f-droid could stand to add a directory for known-good repositories, or listings for apps that pull from such repositories without requiring you or I to manually type in a url or scan a QR code, but end of the day, its free softwaree maintained by volunteers.
Many apps on Linux work the same way; However, in my experience, once you've downloaded and installed an application's .deb, tar.gz, whatever, it will offer to add its repositories to your package manager's sources, whereas on Android, once you install an .apk, it stays that version until you install a newer version manually or let the Play store or whatever over-write it.
