We're omnivores not carnivores! It's the variety that helped give us the competitive advantage through history. We've also, over time, lost the ability to produce certain chemicals naturally as we get them from our diet...such as vitamin C. Nearly all other animals can produce it. So Vitamin C would be missing from your diet if you only ate meat and you'd die from scurvy. So yeah you wouldn't be smart to only eat meat now but you wouldn't have been smart to only eat meat millions of years back either.