It's important to note that in the context of these Apple features, "ID" is for "Identification", not some kind of "User ID" used as a Username.

Username != ID.

> Your face is now your password.

That's a direct quote from the product page.

Yet they behave like a password.

No they don't. They behave like a username/password combination. A username is an individual identifier and a password is a confirmation that the identifier is valid for the person it's identifying. TouchID and FaceID confirm both - that it's the correct user that has access and that the user is who they say they are.