And where exactly do you think cattle and chickens and pigs get most of their calorie? Answer: vegetarian sources. Grass, grain, crap veg, and food waste from nearby breweries and such.
Where do you think the vast majority of calories come from in most of the world? Grain and vegetables. 20% of the world's calories come from just rice alone.
The problem is mostly cattle, and it's not just "energy." They require massive amounts of land, fertilizer, and water to raise compared to even other forms of protein:
https://www.smithsonianmag.com/science-nature/beef-uses-ten-...
> The team calculated that beef requires 28 times more land, six times more fertilizer and 11 times more water compared to those other food sources. That adds up to about five times more greenhouse gas emissions.
> To further put these findings into perspective, the authors also ran the same calculations for several staple crops. All told, on a calorie-to-calorie basis, potatoes, wheat and rice require two to six time less resources to produce than pork, chicken, eggs or dairy.
So that means that calories from beef require about sixty to one hundred and eighty times as much land, twelve to thirty six times more fertilizer, and twenty two to sixty six times more water.
Look at how much rice you can buy for the cost of a 1lb of the cheapest form of beef - ground - and compare the calories per dollar.
1lb of ground beef is a little under $5 and provides at most 784 calories. 156 calories per dollar. 1lb of rice is around 25-30 seconds per pound. Let's round up to 50 cents to make it sporting. 589 calories per pound (also, 12g of protein.) 1178 calories per dollar.
Rice provides almost eight times as many calories per dollar. Now consider that beef in the US is heavily subsidized (because...drumroll please...it wouldn't be price competitive with other food sources!)