There are plenty of carnivores and herbivores on the planet that don’t use fire, clothing, and tools. We have much more the digestive system of a carnivore or omnivore than of an herbivore.
Humans should eat lots of veggies, meat when they can get it, and fruit in season. But grains and legumes were/are mostly poisonous and indigestible without (relatively speaking) modern processing and some people’s metabolism cannot run on them well. Some get quite sick. Pockets of genetic groups (asians/middle east) may do better with grains but they are killing the rest.