It’s a bit more complicated, I think. Main point: You could be unlocked for hours… The passkey specs intend to make sure, it is still you, who has access to your vault then (or rather that passkey in that moment)…
I don’t know how good this source is, but a short search brought me to this website - I think the whole discussion with password managers and passkeys user verification is already there: WebAuthn User Verification & User Presence for Passkeys