In my opinion. If one accepted an answer, he or she would not expect more answers. Why not close this question automatically?
Because many times you can get a better answer, you can get more answers, you can change your mind and decide that one answer is better than the others.
Because the questions are also for [future] readers, which might come from a different context and a different answer might be useful to them as well, or they might have some different idea.
Because this site, ultimately is a site for asking questions and having a repository of plausible answers.