I'm an introvert, but I am one of those people who does not say "good" or "fine" if I am not actually doing good or fine. To me, it feels dishonest. I don't like lying to people without having an actual important reason to do so. Sometimes, I will just choose to treat the question as a greeting rather than as an actual question, but when I do that, I respond with a real greeting like "hello" instead of responding with a fictional answer to the non-question.