Because it's easy to substitute carbs for animal products and you wind up incredibly unhealthy but "vegan." Big diet changes do take some planning and effort in order to ensure you are eating in a physically sustainable manner.
You can down vote me if you want, but I'm telling you why most people should ask and map out a real strategy. Not just rely on a "2 second Google search" -- most people have a poor understanding of a good diet and health education is rarely offered as a mandatory class. A "2 second Google search" will lead people down a very bad road.