Why, you ask? Because by eating a cow/pig/chicken, you're eating something which ate plants and also used energy in their lives. So you're eating far, far more plants indirectly than you would be by just eating the plants.
Of course there's also the fact carbon emissions would decrease if people didn't pointless breed >1 billion cows. There wouldn't naturally be so many, and there would be plants grown in those spaces instead.
The natural food chain is inefficient, but that doesn't matter to the individual animal who eats another animal, because they will take any food source that they can get. Humans on the other hand are perfectly capable of living on plants and are not bound by instinct. It makes no sense to deliberately take the choice of making all your lives extremely inefficient and wasting resources. It especially confuses me when people take steps like shortening showers to save water, but ignore the effects of animal agriculture.
Also, lol at tomato propaganda driving insects insane.