The logic behind it is that a smartphone-bound passkey represents two factors of authentication: what you have (the phone) and who you are (the fingerprint used to unlock the phone's passkey store).

Anything on a PC is easily copied and can only ever be safely assumed to represent one factor: what you know (the password to unlock your password manager). Thus the benefit of getting a two-factor authentication in one convenient step falls away.

Of course it's still super annoying, especially if you don't really trust your smartphone OS vendor and use a portable password manager already.