Because it is a reality of human psychology that people will status-penalize your current self for the mistakes of your past self, even if they (say that they) try not to. Also, it would be undesirable to have a bunch of new comments of the form "I've changed my mind about that semicolon". Such comments might even be downvoted, resulting in a no-win situation for the commenter. And you're not actually editing recorded history. People know the rules, and know that comments can be edited, so expect that people will try if possible to make all their comments make their current self look good. And if someone quotes your comment before you edit it, you still suffer all the consequences. [...] To prevent a situation where someone is hemorrhaging karma and unable to do anything about it. In general, this a case of "leave well enough alone", or, "if it ain't broke, don't fix it". Editing comments -- like any feature -- has potential for abuse, but it's not currently causing any problems severe enough to outweigh the benefits.

I for one frequently review my comments well after the fact to correct stupid mistakes in the wording which I failed to catch the first time around. When I change my mind after more than a few minutes of reflection, I prefer to create new comments to draw attention to the fact that I no longer endorse the statement(s) made in previous comments, but I do not like to create new comments to point out "This comment would have been worded differently if I were a competent editor."