Well, the ‘something you have’ factor when it comes to MFA and Passkeys are concerned is particularly important given that the additional factor such as a PIN used to unlock the device is not necessarily as secure as a full blown password. As such, the original conception behind Passkeys was to make them tightly bound to some given physical device (i.e., the ‘something you have’ factor) by typically relying on a secure hardware element in the device itself to store Passkeys. Given this, providing a method to easily port the Passkey between devices was not given a high priority.
Imagine a scenario where someone breaks into your password manager or iCloud account and they are easily able to move or replicate a Passkey from a device you possess to one of their own devices. They are now able to log into an account secured by that passkey. However, if the Passkey was set up as a device-bound Passkey it would be impossible for them to remotely move or copy it to any of their own devices. For this reason I personally prefer to only set up device-bound Passkeys and avoid schemes that make a Passkey portable.
