That's because most vegans ARE malnourished. Humans are omnivores, we are evolved to eat meat. The biggest nutrient deficiencies in vegans are lack of essential amino acids that only eating meat (or balancing all protein sources constantly to ensure all essential amino acids are being met) can provide, as well as low levels of fat, minerals and crucial coenzymes. These can usually only be met with supplementation. We should definitely be reducing meat consumption, but until an effective replacement is found (insect farming or lab based culture probably), we will always need some in the diet.