Imagine you ask a question, get some good answers and after some days you choose the best as the accepted answer.
What if days or even months an even better or more comprehensive answer comes up - is it impolite or to select that answer?
For the author of the originally accepted answer that would be a loss of rep. She/he might even complain and downvote your question or at least isn't motivated to answer your future questions.
What would be the fair solution?