hey, hey... chill.
firstly, theres a difference between militant / evangelist vegans and just... someone who follows a vegan diet. theres lots of very misinformed and self important people that push veganism and vegetarianism as a moral higher ground everyone must take - similarly, people can also get very self important about eating meat, and the two groups of people end up just butting heads for eternity.
as for why - for some people, it is a health thing. i cant say i know the ins and outs of why a vegan diet does so well by some people, but it encourages greater intake of fruit and veg, lower intake of dairy-based treats (which is most of them), and requires you to be more aware of your diet so as to get the right nutrients. so like, fair.
and then some people are just uncomfortable with eating stuff that comes from animals. lbr man, milk, when you think about it, its pretty weird, right??? (i drink milk by the pint but its weird). thats a personal decision, yknow? for some people its a political statement, sure. if you dedicate your life to fighting sweatshop abuse, you prolly wont wear nike trainers - similarly, if you dedicate your life to fighting animal abuse / for animal rights / however u wanna put it, youre not gonna go ahead and eat animals. again, personal decision, totally fine
honestly people on both sides of the veg vs meat argument need to chill out... neither diet is inherently unhealthy, and the effect of a single person making the decision to eat or not eat meat has very little effect on the meat industry or the environment. and yknow what? thems the reals.
EDIT:
int it a bit ironic for you to be using that term?