Sometimes people misread a question or answer, or sometimes discussion in comments or edits in other places will clarify something and cause a shift in perspective that will make a person change their mind about a previously cast vote.
However, it isn't possible to change because votes are locked in unless that specific question or answer is edited. It may be that no edit is necessary or even desired, yet the vote sticks...
I can't think of a compelling reason for forcing such a commitment, and this rule would seem to be at odds with continuous improvement. So, why is it this way? Who can decide to change it?