Why do people who don't believe in Astrology...?

One could suppose they respond for the same reason that Atheists respond to religious questions. (This is just an example, not an objection.) It is necessary to balance the truth from fiction, and depends entirely on your viewpoint. It is best to hear both sides of a discussion than to simply ignore the opinions of others. Generally, when one doesn't agree with a topic, one is still inclined to voice their opinion. So be understanding and accepting, and ignore those responses with which you don't agree.