What's wrong with reviving/bumping an old thread on a forum? It seems way more efficient to me to continue on one central thread. I hate common rules like "don't post in a thread that's a year old, start a new one". Why not keep the conversation in one place? What am I missing?

Isn't this already how we (properly) do email threads? I always go back to find the last email when I send a follow up to avoid breaking into two different threads with two different sets of replies.