because meat is mostly fat and protein and is thus calorically dense and filling, and has a bunch of nutrients that are really hard to come by in plants (eg cobalamin)
if you exclude meat (as well as eggs and dairy) from your diet, you’ll absolutely need to take supplements to cover those nutrients… and they’re usually made from animal byproducts anyway
like, humans are omnivores. you’re supposed to eat plants and meat alike. cutting meat out might be a good choice morally (after all, what’s the point of eating at all if it brings you mental anguish?) but it’s not like you’re physiologically a different kind after that
upd. not gonna spam replies but i wasn’t talking about whether or not calorie density is good nowadays, i was just talking about why humans eat meat at all