They can and are working on it/have already done it. The key then is to make it in large quantities. One way to avoid needing to make the protein is to deliver mRNAs for the heavy chain and light chain of the antibody. This would be faster, but it is not a proven technology so it's unlikely to work anytime soon (<1y) so convalescence treatment is much easier to do since blood transfusions are super common and well established.
