Because in almost all cases, it brings up an old dispute that wasn't resolvable in the first place, and that certainly isn't resolvable now. You know what I'm talking about, Greg -- people adding comments as if the original poster were still there reading the thread, and even though whatever point they're making has already been made a dozen times. The argument that "forums encourage it" isn't accurate: forums encourage people to contribute to ACTIVE threads if they have the same question, problem or issue. That makes perfect sense, because it avoids the problem of having half a dozen active threads on the same topic -- but I've never heard of a single forum that encouraged people to dig up old moldy threads, drag them back onto the front page, and answer a question that was answered a dozen times six years ago (which would accomplish the opposite effect).
On the other hand, sometimes it brings up new slants on an old topic - something newer, anyway than the classic "this is an old thread" comment.
Anyway, if bringing up old topics again is verboten then...most new threads probably will never get started.