Each have his own reason, but I refer you to the definition of veganism by the Vegan Society (whose founder "invented" the world vegan):
> [...] exclude—as far as is possible and practicable—all forms of exploitation of, and cruelty to, animals for food, [..]
While ecology and health are cited by some vegans, many (if not most) of them are interested in avoiding unnecessary cruelty. That's why there's a discussion where some people define themselves as vegan but do eat musles and other "nerveless" animals they don't considered sentient. On the other hand bees, cows and chicken are sentient and most of they don't have a lot of fun at the farm.
> [...] exclude—as far as is possible and practicable—all forms of exploitation of, and cruelty to, animals for food, [..]
While ecology and health are cited by some vegans, many (if not most) of them are interested in avoiding unnecessary cruelty. That's why there's a discussion where some people define themselves as vegan but do eat musles and other "nerveless" animals they don't considered sentient. On the other hand bees, cows and chicken are sentient and most of they don't have a lot of fun at the farm.
[1] https://www.vegansociety.com/go-vegan/definition-veganism