“Because each type of monoclonal antibody is carefully designed to work against a specific protein on the COVID-19 virus, a mutation in that viral protein could render the monoclonal antibody ineffective. Indeed, this is exactly what happened as different COVID-19 variants have evolved.”
Like the annual flu shot, almost useless.
Monoclonal treatments are very effective, but have a limited "shelf life" as the target they can neutralize morphs to escape the antibody. They are difficult to produce and a financial risk as the target may morph before the cost of making the monoclonal product can be amortized.