If you see a user who is resurrecting posts at an excessive, and obnoxious level, please send me a cc mail.
People are correct about SEO and not closing threads in general But there are other reasons.
Some of these threads have practical use for ministering to people, and keeping them open allows users to engage in needed discussion when the situation arises. Some new users may not want to make a new thread about said topic, or may feel intimidated about making a new thread as a newb. Sometimes people find a topic through a google search, sign up, and they might not be able to find the start new thread button due to lack of familiarity with CC.
Other times, keeping threads open can prevent over clutter of threads on a certain topic.
Sometimes closing someone's thread can lead to the drama of the century!
Back when the forums were in their infancy, I would close a few old threads. This was before the policy of keeping most threads unclosed. One of the users, a MUCH significant time later tried posting to one of their closed threads. This user viewed it as a personal attack of sorts, even after an explanation. There was much carrying on and drama, and oh my goodness. You wouldn't think such a thing would cause that level of emotional trauma, but it did.
Even after the explanation.