Why are role-playing games held in such distain?

I’ve played Dungeons and Dragons, among other games, since I was six. Usually, when I mention this to someone, I get a reaction somewhere between confusion, amusement, and mild scorn. Occasionally, someone will be genuinely interested, but mostly they write the hobby off as a sign of irredeemable nerdiness.

So here’s the question: why? Why do people automatically assume that role-playing games are the purview of socially inept weirdos who live with their parents? Of course, not everyone does this, but a surprising majority still do. So, why is this impression so fixed in people’s minds, and (this is important) what could be done to correct it?