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.

Because the feedstock for the cultured meat comes from plants. Eating plants is going to be more efficient than using plants as feedstock and using energy to turn that feedstock into meat. Cultured meat will be more efficient than raising the whole animal, but it can't be more efficient than its own feedstock.

There is one way cultured meat could be more efficient, but I think it's a ways off. Companies like Solar Foods and Air Protein are using hydrogenotrophs to make proteins and carbohydrates from CO2, with no need for plants as input. So zero need for arable land. The process will still need energy, but on every other metric I think it'll be more efficient than even plants. And per Jim Mellon's book Moo's Law, they'll be able to make feedstock for cultured meat as well. But neither of these companies are to market yet. It'll be interesting to watch play out.