Not all monoclonal antibodies are the same, and they are expensive
There are three monoclonal antibody treatments available to treat COVID patients (Regeneron (Casirivimab, Imdevimab), Eli Lilly (Bamlanivimab and Etesevimab), GlaxoSmithKline (Sotrovimab)). However, they are not all effective against the current variant - Omicron. It appears that GlaxoSmithKline's monoclonal antibody treatment may be the only effective treatment against Omicron (see Washington Post article).
Additionally, these treatments can be expensive. While a dose of a COVID vaccine costs approximately $20, a dose of Regeneron's treatment costs approximately $2,100 (see CBS Marketplace article). Currently, the US government covers the cost of these treatments. However, given the more than 100 times expense for such a treatment, perhaps patients receiving the treatments and having refused the vaccine should reimburse the government.