Stop thinking of them as alternatives to passwords. That is something they do, incidentally. Really, they are an alternative to normal TOTP 2FA (and shudders SMS 2FA). Those were already dependent on an app on a single device, or a password manager. And now, you can have the security of that, automatically used with biometrics. It is only because they are so secure, due to being a cryptographic key, that they can replace passwords.

They really should come up with a way to transfer them across devices/password managers though.