are not supposed to? the fact that a human can do something in no way addresses the question of if it should be done in the first place.
veganism is about MUCH more than a mere dietary choice-it is a moral imperative which rejects violence and exploitation.
you said it yourself, humans can live healthy, in most cases more healthy, without eating animal products of any kind. if one can live one's life without intentionally harming others, why would they choose not to do so?