I think threads that show up show up because people still have something to say about them (plus there are always new members joining - unless you read the boards every eight hours, you will miss many topics forever, unless you have time to dig through past the first page of posts).
I do admit it annoys me when I bring up a thread that has been discussed before, or when I "ressurrect it" by posting to an old one, and people just respond to it to point out that it has already been discussed before, or even worse for the old threads, to make snide comments about bringing back a thread from the "dead." Seems like a big waste of bandwidth to me.
In any event, I think the topics that resurface do so because they haven't been exhausted - there is still something useful to say. The really meaty topics probably will NEVER be exhausted - they are the things you can be sitting on the porch in the rocking chair, your teeth in the glass next to you, talking about until you shrivel up and die. Those tend to be the most interesting discussions, too, because you can really dig deep into the topic and get to its soul. Sometimes threads go on for a long time before that really even begins to happen.