This is an interesting question with no obvious answer, although one could postulate at least a few reasons why former presidents wouldn't want to run for office. Bill Clinton, for example, has expressed his belief that his post-presidential successes in facilitating positive social change on a global scale would simply not have been possible to achieve had he been forced to overcome the constraints imposed by the obligations of political office. It would be difficult to deny Clinton's basic sentiment that the powers afforded by political office are significantly limited by the various obligations expected of those who hold the office. With that said, Bill Clinton's decision to stay out of the political arena hardly seems inevitable. When Hillary Clinton ran for the Democratic Presidential nomination in 2008, many suspected that Bill Clinton was poised to become her Secretary of State. After all, when in 2009 Obama's diplomatic efforts failed to convince North Korean authorities to release two imprisoned American journalists, it was Bill Clinton who got sent in to clean up the mess. We obviously can't know whether or not Bill Clinton would have accepted a political post had it been offered, but there is some precedent for former presidents re-entering the political arena. Howard Taft, for example, became Chief Justice of the Supreme Court eight years after his presidency ended, but in this sense Taft was a political anomaly. If I were to offer an assessment based largely on intuition, the duties and obligations that presidents are expected to uphold take a tremendous toll not only on the president but on the president's family. The demands of the office are unforgiving and unsustainable bordering on cruel and unusual. Once you factor in the average age of those who have endured the experience, it seems difficult to imagine why they should feel inclined to continue subjecting themselves and their families to a life of relentless and merciless public scrutiny once they are presented with opportunities to do virtually whatever they want.