We have archives, and move useful threads there for later reference, but we do not--and I don't think we should--move every single thread over. So it becomes a guessing game--who's going to drag up what?
And, I have to say, in addition to this problem, it's just as much an issue to start a new thread on a really old topic--for which there isn't anything new to say, and it just riles people up. Not a trolling per se, but, for example, the merits of using any of the big box stores as yoiur primary LFS, or a bad experience at one of them--nothing new, the starter gets to vent about their particular experience...But it really isn't new or of value to the vast majority. Yet these pages hit 5+ pages often.