In best practices for SSH keys you should be using different keys on every device and shouldn't be extracting/copying them. Best practices are also to use different keys for every SSH host in addition to per-device keys.

Passkeys enforcing keys not be extracted/copyed as plaintext and that keys are per-host/domain are just following ancient SSH best practices.

Because yes the SSH world is sloppy with dangerous plaintext (or modest password encrypted) key copying and reuse. That does make it a "simpler" ecosystem, but that doesn't make it a better ecosystem.

That said, Passkeys generally are frequently copied, most of the ecosystems support multi-device sync and backup. They just aren't done in a way that provide plaintext export if you want to print a book of your Passkeys and store it in a safe deposit box or sloppily `scp` them from one machine to another.