Tarm: People are fickle. Internet even encourage them to be like that. So people doesn't just have a look at a community and think "Hey I'll start reading a lot of thread for months because it's so fun and I have nothing else to do".
JDelekto: It could be that people just want to make sure they don't repeat ancient history. I suppose it depends on the forum topic, but new folks don't tend to read the posts meant for new folks.
It's kind of embarrassing at times to post something that's already been common knowledge to the forum denizens. I find myself at times, before replying, to at least go to the beginning of the thread and make sure I don't add a "me too" reply.
That's my basic move if the thread isn't too long. Check if its already been said. I get lazy sometimes though. ;)