after a certain time has elapsed the ability to edit is removed. it is assumed that after a few months there should be little reason to edit posts. Most of the time the reasons if they exist are bad ones (i.e. covering tracks) while in other cases they are quite innocent intentioned (like an update). If you need an edit beyond the cutoff report the post to a moderator and include what you want changed. They can modify the post for you if there is good reason.