Android syncs them to your Google account and iPhone to your iCloud account by default. Which isn't a perfect solution but, again, is pretty good for most people.
This is without 2fa enabled on my Google account.
I'm also pretty sure I don't have any accounts that can ONLY be accessed via passkey.
Maybe because your kid was playing with your phone and kept entering the wrong passcode and now you’re locked out for several hours?
Or because Apple detests anyone else touching your phone and you’re traveling internationally and your screen cracked and you took it to a local repair shop which in the process of replacing the screen triggered something Apple didn’t like and you’re locked out for a decade.