Forum questions are not only for the people that ask the question to find the answer but also for people with the same answer finding good information about the topic. Leaving a thread open when the question already has been answered lowers the amount of good info that a thread has regarding an issue, making it more difficult for people to find the answer to their (similar) question. When a question has multiple answers like 'what is your favourite colour and why?' extra replies do add to the conversation, but with 'How do you make a sword?' there is only one answer.
If you want to 'hang out' a thread should be a discussion, not a question.