For the job I work at, I easily have 100-200 accounts that I need access to (we have multiple environments for the same projects, so things tend to get inflated very quickly). The accounts that I need to access are accounts that I did not make, and do not have admin control over. If I didn't have a password manager that someone could "share" these passwords to me, I would instead have to do some wonky method of:
Employee sends me two emails encrypted. Email 1 contains a list of usernames. Email 2 contains a list of passwords.
Any time I need to access any site, I need to open up the two files that were sent, count down to whatever line that project would be relevant to, then copy/paste the username from the first file. Then do the same with the other. (And hope that I didn't miscount.)
This also doesn't account for things like password expiration. Some of our projects require it, while others don't. I would constantly be receiving new encrypted emails with this info from people, and be struggling to keep the locally stored files updated.
This would then also open me up to higher risk in that, if my computer ever got compromised, or anyone elses for that matter, than then every single password would have to be changed for every single site, and new encrypted emails would have to be sent out.
It's stressing me out just to think about, honestly.
Password managers are, honestly, kind of a clunky and weird idea. I think that the sorts of problems we see are likely always going to be an issue, but until we get passed the entire idea of even using username/password as the standard, than we're going to need a means to manage them in this way. Basically, I wouldn't call Password Managers "secure" but I'd call them "secure-ish". It's making the best of a bad situation.