A passkey uses FIDO2, which asks you to sign a challenge. If you use OpenSSH with a security key, it will... use FIDO2. If you use OpenSSH with a private key on your computer, you also sign a challenge, right? So it's not less complicated.

WebAuthn just adds a few things like the relying party and a counter (that nobody seems to use). And the relying party helps preventing phishing, which SSH doesn't do really well in practice (most people don't use SSH certificates and don't check the server fingerprints).

So it's just not true that passkeys are more complicated to do the same thing.