Because somebody reopens a thread which was a good one, and popular, and something new has come up which makes it pertinent again. Say, the jury comes back with a verdict on a big case and the thread hasn’t been updated in six months.
There was also a discussion here in which it became clear: There was no general agreement on the definition of what constitutes a “zombie” thread. It was more complicated than simply how long since the last post. There was also argument about how old a thread should be (from the date of the OP), and various other criteria that people tried to suggest.
The question was not about locking old threads, but simply displaying them in some way to make it really obvious, so people wouldn’t resurrect them accidentally. The idea was simply to display the date of the thread (date of OP? date of last post?) is large red bold-face, or something like that. And there wasn’t any general agreement on even that.
In General Questions, and other forums, sometimes people may post pertinent new information to even very old threads. And sometimes when they are revived other posters will add additional new information.
In general, I will close a zombie thread in GQ mainly when the new post is pointless or adds no new information.
Just search for something fairly specific which interests you, like “Gilligan” or “Reinheitsgebot”, then scroll down a few pages and you’re in zombie-land. I’m thinking a lot of these zombies are caused by first-time posters who found the thread on a Google search and have no idea where they are.
Personally, I have far more often been annoyed by having a zombie thread locked on me (sometimes during the time I took to craft a reply to some old or new post in it), than by it not being locked… In fact, I have never been annoyed by a thread not being locked.
I expect that, when newbies open zombie threads, they have come across said thread by doing a google search in which the Dope thread was high on the results list. They then click that link, read a little, and decide to reply.
I used to oppose zombie threads, and while I still find them vaguely irritating I no longer think they should be banned. It’s not friendly to possible new subscribers to automatically chastise them for not knowing the board culture.
I’m not saying zombie threads should be banned or anything.
I guess my only beef with them is when I accidentally post in a really old thread that someone else has resurrected, making me look perhaps slightly stupid. That’s all.
We disallow zombie threads in some forums (like the Pit), where no good can come of resurrected hurtful stuff that had faded out years ago. In other situations, we basically look at whether the resurrection has (or could) provide something new.
The best solution is to read at least some of the prior posts, and make a habit of noticing the date. And don’t worry about it, we’ve all posted to zombie threads, so there’s no reason for embarrassment. (Anyone saying “Nyah, nyah, you posted to a zombie thread” will suffer the curse of themselves posting to a zombie thread within a few weeks.)