It’'s really up to the end device (and the user of said device) to decide how much security to put around the local keys. But importantly, it also requires access to the device the passkeys are stored on which is a second factor. And notably many of the implementations of it require biometrics to unlock.

The “one password” thing is also true of password managers, of course. One thing about having one master passphrase is that if you do not have to remember 50 of them, then you can make that passphrase better then you otherwise might, plus it should be unique, which prevents one of the most common attack vectors.