The operating system doesn't manage attestation because that's totally useless for the stated goal of the attestation system. Enterprises don't want their SaaS vendors to accept passkeys from some random employee's BitWarden, instead of the hardware keys they issued the employee. If the OS manages attestation and doesn't send anything to the relying party, then it doesn't solve anybody's problem at all.
Also, as I understand it, sites can whitelist credential hardware.
If not, then the attestation is security theater. I (or an attacker on your machine), can just make a sw emulator of a hw attestation device, and use that to protect my choice of OS, (and skim your credentials).
If a whitelist exists, then my “hijack your OS” plan works: Require the builtin macos/windows/signed chrome on signed os password managers. That’s 90% of the market (and dropping) right now.
It talks about whatever you used to authenticate and the platform can manipulate (or omit) it.
Yes, and iOS and Android's Passkey implementation does not support it, since doing so would be lying about a given credential being hardware-backend when it's actually not (due to being cloud-synced and often recoverable via some process).
Attestation is only for hardware authenticators, either dedicated ones like Yubikeys or non-synchronized Android WebAuthN credentials. (iOS only supports them in MDM contexts anymore, I believe.)