Re-using passwords with a unique login would indeed be stupid because it would not only be weak against attackers but also have terrible ergonomics: it's usual

Re-using passwords with a unique login would indeed be stupid because it would not only be weak against attackers but also have terrible ergonomics: it's usually much easier to regain access after forgetting the password than to regain access after forgetting the login. Source: I often forget both of them. What I and many others do is reuse not a password, but reuse a password formula (in my case with slight but easily memorable variations depending on importance, or if forced by stupid password rules). If you saw a dozen of my email/password combinations in the clear you would be able to reverse engineer the rules and then guess combinations for arbitrary sites almost as good as me. It's a calculated risk, just like trusting a password manager is a calculated risk. Right now I consider password managers the better trade-off, but still only slightly better, by a margin small enough to make it not worthwhile to invest in a habit change. If the "formula" I happen to use was just a little harder to reverse engineer than it sadly is I would consider it strictly safer than password managers.