Sometimes, the thread may have gone in a direction the OP finds too distasteful, hurtful, etc. to discuss. I do think in these situations the OP should add a response to the thread stating that they won't be back, not a "Goodbye Cruel Thread" but just a heads-up so posters don't keep asking the OP for clarifications or updates when none will be given.
Sometimes, the OP may simply forget to update the thread. Other times, the OP may not think that the update is necessary--I've done that before. In a thread in All in a Day's Work I asked for advice regarding an employee, and honestly didn't think anyone would be interested in reading an update.
I did provide one when requested. A few months later, the situation ended, but I figured at that point everyone had forgotten/would no longer be interested in an update, but a poster did ask for one awhile later, which I provided.