Imagine you switch from iOS to Android, now you need to create a new one for every site that need uses passkeys.
Not to mention the hassle if you forgot one.
In most cases it is as simple as: Download the Apple Passwords app on Windows and use it to login to that site on Chrome and then add a new "Google Passkey" in Chrome. It is a hassle, and the UX isn't entirely there today to make it feel "easy" or "seamless", but it is a simple hassle, relatively.
No that's just assuming the 70%+ case "average part of the bell curve" where a user switching iOS to Android already has a Windows machine somewhere. Remove the install step of the Apple Passwords app in the rarer case that the user has a Mac or iPad as their primary "other device".
(If the hypothetical user was using Linux machines already by that point, they wouldn't have been using iOS Keychain for their Passkeys in the first place and probably would have the necessary skills or tech guru friend to load up Apple Passwords in a WINE session or in a Windows VM/dual boot.)
All else fails, there's always most sites will continue to (forever) fallback to email-based recovery because we should all face it that email accounts have been the average user's primary password manager for decades anyway.
