We're just talking about software delivery here. Its the same as Debian not requiring you register before using `apt` to install packages (or every other linux distro).
It depends on context, but often having an object representing the user is at best a speed bump to a bad actor. Social media's an obvious example here. I know Twitter does quite a lot to limit bad actors, but the outcome is still not great.
The vast bulk of sites want to make signup easy, meaning user objects are cheap. Cheap user ids are easily disposed of and replaced. So if you need to keep bad actors out, user accounts may not help a ton.