
Economic theory suggests that as incomes rise past a point, people choose to 'purchase' or consume more leisure time which produces a backward bending labor supply curve (here is the Wikipedia graph which is nicer than the one I came up with).
It seems to me that one could spend six years fighting for, and then performing the duties of, the Oval office, or one could relax, travel, read some good books...you know, enjoy life. I know what I would choose.
I suppose being the most powerful person in the world might have something to do with it. And I don't mean that as a snarky comment about Hillary's ambitions. The President of the United States can affect hundreds of millions (even billions) of peoples lives. The ability to do (what one believes to be) a lot of good in the world could be pretty strong incentive, I suppose.
By the way, for those who don't already have $100 million, it appears the Presidency is a pretty good cash cow upon retirement...