With a password manager storing unique passwords, your password is still being sent over the network to that website each time you log in.

With a password manager storing passkeys, your private passkey is not transmitted as part of the website request. It can't be intercepted or accidentally/negligently stored in plaintext in a database or server logs.

You still have to trust the secure syncing of your passkeys, just like you do with passwords, but there are still fewer threat vectors than with passwords.