This is Apple's implementation, not Passkeys per se.
I was confused by this when they announced it, because I couldn't think of any reason why you'd want cloud sync for device keys. And it turns out, you don't.
Apple uses "sync passkeys", not device-bound passkeys, and uses iCloud to distribute them to your other iThingies (but of course not other platforms).
I won't use them either, for the same reason. I refuse to sync authentication creds - I will not store them on something I don't control. And even if I were to do so, Linux would need to be included for it to be useful to me.
Anyway, yeah, it Apple didn't need to require cloud nonsense, but they chose to. I understand where they're coming from, I just don't like it and won't do it.