It's been [proposed any number of times][1] to allow moderators to accept an answer on behalf of a user. This has also been shot down the same number of times.

The rationale is that accepting an answer is meant to indicate which answer was most useful to the person asking the question, while community consensus as to the "best" answer is expressed through votes.

Thus it can happen that the accepted answer isn't the highest rated answer, or even is voted below zero.

It doesn't really matter much that a question doesn't have an accepted answer; so long as it has at least one _upvoted_ answer it will not count as an unanswered question.


  [1]: http://meta.stackexchange.com/q/18312/189912