You want the real answer? I'll give it to you:

The real reason is because about 85%-90% of the people who use this site are 18-25 yr old guys who spend a lot more time in front of their computers than they do getting laid and being normal. This leads to a major back-up of semen and hostility.

Also, these types of guys probably got picked on a lot in high school by athletes and more popular guys, so a chance to lash out at the world under the cowardly blanket of anonymity is very appealing to them.

The bottom line is that anyone who just acts like a dick in general is unhappy with himself. People who are secure and confident in themselves don't behave this way. Don't let it bother you. Be thankful you're not one of them.