Actually most good writers ( as in book authors ) are FE-doms (Feeling Extroverted) which makes them extraverted and not introverted. They are good at it because they are naturally good at consciously think and reflect about how they affect other people and how other people affect them, which is good for making believable characters.

It still amazes me that people on this board stick to their (hero) introvert/extravert dichotomy instead of actually researching the subject which you would expect engineers and techies to have an inkling to do. Search words, MBTI, Keyser, Jung, Big Five.