To my knowledge its easy to do this with proteins that are directly coded for in our genes. For example there is a gene for insulin so we can clip it, transfect it into a cell culture and get those cells to crank out a bunch of insulin that we can extract, purify and make into a usable, stable product. The problem with antibodies is that we need to be exposed to an antigen capable of stimulating the development of the corresponding antibody. There is a very complicated set of receptors that capture the antigen, internalize and process it to present it to cells that will modify a whole bunch stuff (Thats the limit of my personal knowledge) to generate an antibody capable of neutralizing that substance. In short you still need a living being with some sort of immune system capable of processing that compound and generating an antibody to it, its unbelievably complicated. The good news is once that cell starts cranking out antibody it starts to clone itself so we can extract it, make a hybridoma (fuze the antibody-producing cell from a live animal with immortal cancer cells to create an immortal hybrid that cranks out the desired antibody) then we can continue with the process in cell culture as per usual. Its just cheaper and easier to use live animals with high titre to that antibody. Source: I’ve taken a couple immunology courses and my lab specializes in immunohistochemistry so I’m quite familiar with antigen/antibody interactions but not so much with antibody production.