Storing passwords in a password manager is storing a shared secret where you can only control the security on your end and thus is still vulnerable to theft in a breach, negligence on the part of the party you’ve shared it with, phishing, man in the middle potentially, etc.

Storing a passkey in a password manager on the other hand is storing an unshared secret that nobody but you has access to, doesn’t leave your device during use, is highly phishing resistant, can’t be mishandled by the sites you use it to connect to etc.