The main reason why you can't edit a post after a certain period is to stop people altering a post after others have made replies referring to it. In some forums (not here) I've seen several posts that have been made almost meaningless because the original poster has returned, in some cases days later, and changed the post completely.
It makes sense to allow editing for a short period as it's very easy to miss typos and so on, even with the 'Preview Reply' button, until the post is actually made. In another forum (where I'm an Administrator) we only allow 10 minutes for editing.
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To err is human; to arr is pirate.