To answer your question: because most people reuse their passwords across websites. I don't care if you can log into my HN account. I do care if you use that sa

To answer your question: because most people reuse their passwords across websites. I don't care if you can log into my HN account. I do care if you use that same password to log into my Gmail account. You can argue that people shouldn't reuse passwords all you want, that is just reality.