Sticky matter, age. Reagan was pretty sharp for a short while, then he got all soft, and probably had some dementia by the time he left office (remember the shot of him fielding questions from reporters and getting really stuck, only to be saved by Nancy muttering words to him under her breath).
Warren Buffett is as sharp as can be. But he, I believe, would not have the mettle to serve as commander in chief of our armed forces and the leader of the free world. The job would just waste him. Just remember what Bill Clinton looked like when he got elected:
Plus he needed bypass surgery a few years later.
Here's W in 2000: