Comments are supposed to be a lightweight mechanism to request clarification or otherwise suggest improvements to the post. They are not meant to be maintained as part of the canon of the answer itself.
As such, comments do not have the accountability and revision tracking of regular posts, so if a third party were to edit somebody's comment, there's no way for anyone to know what they edited or even that an edit has been made at all (i.e. there is no bumping of posts when a comment is edited).
At the root of what makes SE work is accountability and peer vetting. Comments are supposed to be a lightweight commentary mechanism, so we don't want to burden that system with the history tracking and user notifications needed whenever someone goes in and changes the words of another user. That is outside the purview of how comments are meant to be used.