Sure, but those same arguments apply to synthetic meat. If you're processing inputs in a lab, you can likely be far more flexible than how a specific animal's digestive system happened to evolve, and you don't have the overhead of running a cow for 1-3 years¹ before slaughter. From what I can tell, about 75% of a non-lactating cow's energy input goes into body gain. So that's 25% off the top before you even consider what goes into skin, blood, bones, and the less palatable organs. Apparently, beef yield makes up about 40% of a cow's total mass, so assuming uniform energy usage for all body parts (which could be wildly wrong), only around 30% of the calories actually processed by a cow will turn into beef.

¹And having searched for that statistic and read the necessary surrounding material, I need to lie down for a while. Jesus Christ.