People also adopt religions to be fashionable, and they don't follow all the tenets of that religion. Seems bad-faith to ask someone who professes a moral stance why they don't behave more like someone who is just professing that moral stance to be fashionable.

Who is asking that?

You might be surprised. Imposing such a dietary restriction can take a lot of mental and emotional effort (especially if people around you are annoyed by the fact they must accomodate you, or act threatened by your choices). When I started being veg, I had very few (0?) other lifestyle changes unless you include maybe trying make a few environmentally conscious decisions. I wouldn't take your word on the motivations of your friends, unless you know they are no longer veg today.