Just about nobody logs passwords on purpose. But really stupid IoT devices accept credentials as like query strings, or part of the path or something, and it's common to log those. The attacker is sending you passwords meant for a much less secure system.
You probably shouldn't log usernames then, or really any form fields, as users might accidentally enter a password into one of them. Kind of defeats the point of web forms, but safety is important!
A password being put into a normal text field in a properly submitted form is a lot less likely than getting into some query or path. And a database is more likely to be handled properly than some random log file.
