Already been discussed many times; don't hold your breath.
The reasoning is that a long time ago when we did have the option of editing our forum posts, people abused it by going back and changing what they'd said in their forum posts, much to the confusion (and sometimes anger) of other people who had replied to that post.
As I mentioned elsewhere, I'd like to see a "Preview" button. At least that would give us another opportunity to review what we've written before it's set in stone. From what I understand from Staff, a Preview is not high up on the priority list of things needing doing around here.
If you accidentally post the same question more than once, just tag it as a duplicate thread (see tags on the sidebar) and post the link where you want to receive replies.