|laziness, impatience, and hubris
As a sidenote (if you're into this), evolutionary biologists try explain it in a similar way, but they invoke 'reciprocity'. The arguement is that animals (and humans) only do favours for each other if there is a chance it will be repaid. For that, you need to meet the person/creature you did a favour for, and be able to recognise them. But the critical point is that you are likely to see them and deal with them a second time.
In ordinary discussion groups, you can change your email address and .sig and you are a different person. So there's little reason for favours, since they aren't likely to be returned.
But around here people are attached to their names, and would rather behave than give them up.
Please don't confuse me for someone who actually believes this gak, I've just been pumped full of it for many years. Personally I like participating in flame wars because there's no chance of me getting hurt.