Questions and answers can be edited. Why votes should stay fixed after some time?
I find that the main importance for votes is to give information to the future reader about the question and the answers. By voting questions can be organized (roughly) by popularity and this popularity reflects (roughly): How well written it is. How important it is. How many others have had that question.
Likewise, votes help organize several answers to a question according (roughly) to popularity, and this popularity reflecting (roughly): How useful it is. How well written it is.
Addendum: Of course, an edit is not the only reason for a vote change. As well as editing an answer can have the purpose of fixing a mistake. So can be a change of vote.