SSH is nice because you don't have to think about it. Your private key sits in your .ssh folder, and then everything is transparent. You _can_ put an SSH key in a smartcard if you want, but you have to opt-in to this kind of pain. And even if you do, almost all SSH servers will support that login method without issue.
Passkeys don't sit in your .passkey folder. Your browser doesn't look for passkeys in a standard folder at all. You don't just do passkey-keygen like you would ssh-keygen and forget about it.
Websites might support various combinations of FIDO/U2F/TOTP security keys, your USB security key might support various combination of FIDO2/CTAP/WebAuthn, and the user will be left confused what any of this mess means, why there are so many competing standards, and why they're asked to scan a QR code when they plug in their dongle, and it doesn't just work at all.
The attempts to restrict when and how they are stored, and how you can access them - those are going to cause a lot of pain and confusion.
I have all of my SSH keys stored in KeepassXC, which (imho) is a lot more secure than having them hang around in my .ssh directory. Open KeepassXC, and the keys are available. Close it, and they're gone. Synchronizing the KeepassXC-file across devices means that I have access to the keys on all of my devices.
The big companies pushing passkeys are trying very hard to prevent this kind of convenience.
We've been storing ssh keys directly on our yubikeys since before passkeys were a thing.
Not only is it clearly more secure it's also been a usability lift. Plugin your yubikey, start an ssh agent, and run ssh-add -K to get all your resident keys added to your current session.
