Because it is easier and more legal than saying I am going to apply neo trace and come slit your throat, while still getting my point across. Until you realize the staff are usually too busy massaging their own erect egos to do anything or really care about it anyways, so you end up flaming back and getting in trouble despite the fact that the only way to finish a troll off is with acid or flaming, and flaming is free, where as acid is best used for tie-dying T-shirts.