This is partly because so many things want an account now. I have over 500 passwords saved, it would be straight up impossible to remember unique strings for each site.
I'd argue that it is not just about remembering passwords. A password manager also helps you remember that you even have an account. I have a similar number of passwords stored as you and there's no way I'd remember all the sites I signed up for (never mind the passwords) if not for the password manager I use.
If I was diligent I could probably track them via confirmation emails (or self-authored "confirmations" for services that don't send a confirmation email), but I can guarantee that a lot would slip through the cracks if I were to attempt that.
A password manager also gives you a convenient place to store (and share) secrets, recovery keys, SSH keys, and similar bits of security related information that you cannot memorize.
If you’re actively under attack no Password Manager, mental algorithm/ password pattern, Yubikey, or MFA will prevent someone from just using your authenticated session(s).
Does that mean we shouldn’t use these mechanisms? Of course not. When the risk is only realized with full compromise—saying XYZ could pose a threat is moot from a security perspective.
ok but that also is prone to a weakness in any part of that chain assuming you even set it up properly in the first place. each piece is another layer that can be hacked or improperly setup.
How many of those accounts are essential?
I create throw-away accounts on the regular.
There are many accounts, but few that really matter. For the ones that matter I take care to make sure I have the passwords. For the rest, who cares :)