If the first post is deleted, the entire thread is automatically deleted.
If the first post is substantially edited, it can turn all subsequent posts into gibberish and make those authors look stupid.
If a member is having a bad day, decisions they make can both annoy other members and wreck threads. Basically, you need to decide if your posts says what you meant within a few minutes (or if it should be there at all), so that others are not inconvenienced, because a forum like this is a group concern not just about the OP, and also very busy.
On some forums the edit limit is set to one hour (or less), and it needs to be - otherwise threads are completely wrecked. The edit time is extremely long here - too long perhaps, as it can make work for others - but it seems to work as the
mods do not complain about it. Self-deletion of posts (and therefore threads) is not really a viable option. Anything can spark an argument round here, and post/thread deletes would be a guaranteed firestarter.
On the other hand it might be fun to introduce an option in The Outside whereby members could delete a post by another, maybe by 10 votes or something. It's mayhem in there anyway...