Actual Carnivorous animals (like Lions for example) become sick when only eating Steaks - In the wild, they usually get Veggies and nutrients by eating the entrails etc of Herbivores (and they eat those FIRST, that's how important it is). Humans are Omnivores, and thus by Nature require these even more. This is actual proven Biological Science.
There are always Exceptions, but that means they are outside the Norm.
(#JordanPeterson sure is a mix of Agree/Disagree)