A common reason for this is that people don't ReplyToTheWholePost
Another common reason is that many people just don't read the whole post, they just scan over it and then respond to what they think is being said. Also, many reasoning tools are not understood by everyone: some people are just incapable of understanding any abstractions; others just can't apply any kind of parable or hyperbole to the situation. Others are just ignorant of the facts but don't know that: e.g. they are sure that the rotation of the planet causes gravitation. Sometimes you just have to ignore a response and trust others will come to the same conclusions as you do. Last, but not least, you should never respond in anger or even try to defend yourself: DefendEachOther.