Three main reasons, I suspect:
This can alienate people who are intrinsically sincere, or who don't realise that I'm not. I've been known to use inappropriate language and blithely assume that listeners will realise that I don't mean it, though I'm trying to stop doing that. It's perilously easy to do wrong, and it makes the arrogant assumption that anyone who doesn't like it just doesn't get it.
I'm very dismissive of people (though I hope I don't let this show too much.) If I meet someone through friends I'll give them the benefit of the doubt, because, hey, my friends have good taste. In almost any other context, my default assumption seems to be that people I meet will have nothing to offer me, and that the onus is on them to convince me otherwise, i.e. 'Fuck you, I've got enough friends'. I rarely make any effort to get to know people at work, big parties, family do's or whatever, beyond the amount necessary to blend in and to keep myself entertained.
You're in or you're out
For some reason, my mental distinction between Friends and Others is very strong. If someone is in the Friends set, I'm interested in their woes, care by proxy about their family and interests, and will put myself out to see them and to help them. With Other people, none of that is true. Their drama just makes me feel uncomfortable, their lives are tedious (unless they generate really good anecdotes), and they merit only enough effort to avoid actual rudeness. I know everyone's like this to a greater or lesser extent, it just surprises me sometimes that I'm so... binary about this.
On re-reading, I'm beginning to think that these could be all be boiled down into one convenient bullet point:
I'm an elitist misanthropic prick
That might be fair, I fear. All I can say in my defense is that I try not to act like it...
As ever, there is a Daily Dinosaur Comic that says it better than I ever could: