For the same reason a lion doesn't just eat plants. Human is an apex predator and an obligate carnivore.
Following and promoting a man-made starvation diet (plants don't have the following nutrients: Vitamins A, D, K2, B12; EPA, DHA; creatine, carnitine, carnosine, taurine; heme-iron, CLA, CoQ10; cholesterol) doesn't make one more conscious. It's a step away from self-love and a step away from nature.