What's wrong with posting in old threads?

[simplechamp]

This is just something I've always been curious about. On almost every forum I've ever been on if someone posts in an old thread there are always comments along the lines of "You know this thread is xxxx months/years old right..."

Why is it frowned upon to dig up old threads? As long as the topic is still somewhat relevant and the person posting has relevant conversation to contribute I don't see the big deal. Sure the OP may have come and gone but the conversation can still continue. So what am I missing here?