> Monoclonal antibodies are, as you say, a treatment. They are not used to prevent infection in the first place. They lessen the severity of the disease but do essentially nothing to avoid its transmission.

You, fundamentally, do not understand how antibodies work.

Just as with the antibodies from a vaccine, if the monoclonal antibodies are in your blood before contact with an infected person you will not get sick. So, monoclonal antibodies can be used profilactically, if so desired.