There are monoclonal antibodies targeting cancer, targeting some viruses (including hepatitis C) and other diseases. Monoclonal antibodies against SARS-CoV-2 were produced within a year or so after discovering SARS-CoV-2. It would appear that scientific world knows quite well how to design and then produce them for a wide spectrum of targets.
Why aren’t HBsAg monoclonal antibodies a thing? My understanding is that getting rid of (most of) HBsAg from blood is speculated to be a prerequisite to cure hepatitis B.