How much do you read threads in relation to how much you post in them? Seeing how there is always more guests lurking than members and also members inside threads but not posting.. it seems like a good number of people are simply reading threads.
So to these people, why don't you feel compelled to post? If I'm really reading a thread, I will most likely make a post in it. To me, it makes the whole process of being on this site more interesting.