Fun fact, you don't need user registration to do that.

But user registration makes it so much easier! Single identifier shared across all user's devices and browsers, and which survives reinstalls, cookie eraser plugins, and needa no GDPR notice - that's the best thing an advertizer can hope for.

Sure, but its a bad metric to decide what sites to use. The evil ones will still track you regardless, the non-evil won't regardless.