Usually because that decision is in within the context of some kind of moral superiority or religious belief (which is in and of itself irritating). In the rare case when it isn't there's also the issue that alcohol is a large part of socialization, someone who's never drank may not have this perspective. I feel I have some credibility on this since I didn't drink for similar ('practical') reasons longer than most people and I think that decision was ultimately a mistake.
I don't drink (and not because I'm morally superior :P) and trust me, it's hard to miss the socialization aspect of it. That's the kind of perspective you couldn't ignore if you wanted to, as people essentially club you about the head with it.
It's weird, you could be hanging with a group of people who are the most socially liberal, equality-seeking types on the planet and they find out you don't drink alcohol and it's OH. MY. GOD. and suddenly they're almost as intolerant as those they oppose.
My decision not to drink is not at all religious or related to moral superiority. It would be silly for me to think I'm morally superior to such a large percentage of the adult population. Regarding socializing, it's definitely there, but not required. I'm often at bars with friends not drinking alcohol.
