Yeah, Fork you too!
It could be because Stack Overflow has thousands of members with (limited) moderation powers.
It could also be because Stack Overflow draws another part of the public than e.g. a forum about some favourite boyband.
And, of course, because this site has so many active members at any given moment plus some smart algorithms to detect possible abuse, and thus abusive behaviour can be stopped real fast.
Plus, we have Meta Stack Overflow, where we can complain about anything that troubles us at Stack Overflow.
Still, peer pressure isn't enough to stop misbehaviour. Consider those Soccer Hooligans who misbehave during lots of all kinds of events, but mostly soccer sport events. Basically, these hooligans are just regular people and there are probably a few at Stack Overflow and Meta Stack Overflow. But they organise themselves in groups for organized misbehaviour. There are online groups of forum hooligans who tend to misbehave at dozens of forums and the only reason why they're not harassing Stack Overflow could be the moderation done by all members. Or maybe Stack Overflow just isn't interesting enough as a target.
Because, strange as it sounds, Stack Overflow might not be interesting enough for online hooligans to misbehave. :-)