They prevent you from being one of these, and copy pasting the password from password manager into the wrong input field. Something that still happens often with many websites not properly auto-filling from password managers.
> They just rely on you being busy, or out, or tired, and just not checking closely enough
If you are "copy-pasting" you are not using your password manager correctly.
It's far too common for websites to redirect to some separate domain for sign in which isn't the one originally used to sign up, getting users used to "oh gotta copy the password again" as a totally normal thing that happens
Password managers rarely are able to autofill 100% of the time. Autofill breaking is not a very strong indicator of a phishing attempt, people are used to manually filling the password in sometimes for totally legit sites.
I'm used to 1Password not being able to autofill, yes. But I'm not used to no account showing up at all when I open the UI panel. If that happens, I immediately know I'm on the wrong domain.
As I said in my comment above, sometimes it’s necessary as websites break the auto fill, or mobile apps don’t offer the password manager sheet.
This very story illustrates how people will override their password manager's builtin protections when panic ensues.
If only everyone did everything perfectly all the time, we wouldn't have any issues!