Herbivores generally have multiple stomachs containing microbes we lack that enable them to get nutrients and energy from what is to humans non-digestible fiber in plants.
Also, in order not to have nutrient deficiencies on a vegan diet, humans have to carefully choose their foods. Although some herbivores also carefully pick and choose the plants they eat, many can just graze over a field. While this is likely not vegan (due to incidental consumption of bugs), it is still a much less varied diet than a human vegan would need to achieve health.
