Pretty much what everyone else said. People used them to cry out for attention, then 'changed' their decision to leave.
I think the staff allows the occasional "I'm leaving" thread to be posted and remain open, but the starter of said thread has to be someone whose departure would affect everybody on the forums (e.g., a moderator or an administrator) or someone who's really earned it by rocking hard enough (e.g., people like Shlup, Spatvark, and myself, not that any of us is leaving in the near future as far as I know).
You're right, Man, even if not that modest.