The meat doesn’t just photosynthesize itself new meat. You need to grow cells on media, i.e. organic nutrients. Those have to come from somewhere. Some of that can be generated from plants (sugar), some you might be able to generate in vats of E. coli or yeast (note these microbes consume media as well), some of that (hormones) is most commonly derived from butchered animals (see FBS). In all cases though, you’re refining a larger mass of organic material into a smaller mass of (more nutrient dense or otherwise preferable) organic material.