plant based animals have long, drawn out digestive systems. ie. ruminents have many bacterial stores to facilitate the breaking down of plant matter.
humans have a short digestive system. singular stomach. short ceaca (appendix). we are very much intended to consume mostly high energy source materials; meats, tubers, and fruiting bodies. leafy greens not so much.
traditionally we are not above eating anything to survive, and thats powerful, especially when we need to operate a energy expensive organ like the brain. gut tissue is also expensive to operate and these two compete against each other for energy requirements.
the expensive tissue hypothesis is worth a look, and evolutionarily supplies a lot of the framework for a more carniverous diet. 😀