It’s not that it is not natural for humans to consume a vegan diet, it’s just more difficult to achieve because humans need different nutrients than cows and deer (who eat mostly grass and plant material respectively). Humans evolved to be omnivores as a survival tactic. They can eat meat, dairy, plants, fungus and bacteria and yeast. But because human beings also developed reasoning, as a part of their evolution, humans can choose to eat a vegetarian or vegan diet because they are choosing not to harm animals, or because they are choosing to eat a more healthful diet on purpose (science has seemed to confirm that eating a more plant based diet is more healthful for longevity and for living a less disease-riddled or painful life).
Back in the cave days, when people didn’t live much past 30, eating meat (or plants or whatever else was most readily available, including Grandma and their own placentas) was probably a very good option, because it has a lot of nutrients and protein. But now, when humans live to be closer to 80, the effects of a meat and dairy based diet can cause a lot of illnesses (heart disease, diabetes, cancer) that wouldn’t have shown up, because the caveman or woman was dead at 30. In those days, eating was merely about day to day survival, nothing more. Before the cavemen learned how to hunt and eat meat, they probably were more vegan-like because they were foraging for roots and nuts and berries. But both groups were still probably dead before they turned 30. Humans evolved to be omnivores to be able to take advantage of whatever was available to them. Today we have a lot of options, including the options to be more compassionate toward animals and be more selective, health-wise toward our bodies.
In today’s world people have different reasons for eating what they eat, because most people are not simply eating to stay alive. Their reasons may be selfish or altruistic or neither.