Because large herbivores can ferment grass or leaves in their guts to make something more nutritious than just fiber and chlorophyll. Small herbivores like koalas and sloths can't do that. >>A vegetarian diet is not grass. True, which is why it's possible. But it's still not optimal. >>What nutrients cant you get with a vegan diet? B vitamins and complete proteins. Also, plants and especially seeds and legumes have anti-nutrients that hamper absorption. Without extensive food prep, most of those would remain in place. Things like sourdough or tofu that ferment the food remove some but not all of these, but a lot of what we eat now is simply ground in a mill and turned into a baked food. You're lucky if you're getting half of what's listed on the nutrition label.