Even if humans were excellent at remembering passwords - I look into my pwd manager and I have over 500 passwords. OK, I am not a common case - I have passwords from various systems, logins, keys, etc. there. But even a common person can have accounts on a hundred sites. And now add rotation requirements. Nobody outside of trained mnemotechnic performers can remember those - and keep them up to date for years. It's not just humanly possible in current environment. Either you use one password virtually everywhere, or you don't remember any of them.