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.