This feels like the biggest misstep that may impede the success of passkeys to me. It's not that surprising, though, since it's pretty rare to find genuine support for multiple authentication factors of the same type already. You can usually only register one number for SMS, and/or one TOTP authenticator, etc. It seems like a lot of sites never really understood (or cared about) the benefits of MFA to the user and so can't be bothered to really understand passkeys either.