Ford is another one that seems completely ignored by all presidents after him. Is the rehab of the presidency the only reason 41 and 43 keep Clinton hanging around?
Carter went off on his own...kind of like a black sheep of the ex-presidents club. No one can force him to play the game. Ford was ignored due to the hostility between him and Reagan after the '76 primaries. By the time the Bushs came to power, Ford was simply too old to do much.
Can you imagine having to spend an evening with Jimmy Carter? It'd be like dining on sour lemon drops.