Q: Someone insulted me. Why aren't you deleting his post/thread?
A: Possible (since we don't have the time or resources to check) insults are also allowed as long as they contain any kind of constructive opinion, info or something else substantial and aren't off-topic. For example, posting something like "you are dumb" will be deleted as it contains no meaningful content. However, if the post is somewhere along the lines of "You are dumb. This is wrong because this website/thread/etc. has explained it's not right", it's in most cases accepted.