Interesting thread. I was once asked this very question,as part of a PHD on Scottish singing style. I think my answer was because I could and there is a buzz you get from singing,alone or in a group. The best answer I heard was from a good friend of mine. For spite,he said. This guy is a mentor and good friend and the most wonderful singer of ballads. Perhaps you have to be cynical and Scottish to appreciate the irony of it. But it made me howl with laughter.