I noticed while looking through the frequent questions page on the main site that a decent portion of those questions don't have accepted answers. I think it is an issue that many of the most popularly searched and well received questions on the site don't have an accepted answers. We could try badgering the original posters of the questions, but it seems that many of them have long since left the site. Is there any way to get accepted answers for these questions or compile the good answers in some way?
There is nothing that can be done apart from contacting the OP about it. As you say many of them have since left the site and are probably out of reach, therefore chances are slim (at best) to get that resolved.
I think there is nothing that should be done either.
I personally think answers not marked accepted are neither an issue nor a problem. The voting system is the better indicator of quality (and even that fails in some cases due to popular opinion).
Sometimes I even find that the system of accepted answers is a nuisance, it is simply a very subjective vote for what the OP thinks helped best. In the worst case that may be a wrong answer. I particularly dislike that the accepted answer is docked to the question, but that may be just me.