- In 1993, George HW Bush was the outgoing President of the United States.
- In 1997, Bob Dole was the outgoing Senate Majority Leader.
- In 2001, Al Gore was the outgoing Vice President of the United States.
- In 2005, John Kerry was still a United States Senator.
- In 2009, John McCain was still a United States Senator.
Neither Romney nor Dukakis ever held a federal position, nor were they in office at the time of their defeat, so it was silly to expect them to show up -- particularly after the scorched-earth bad-blood politics that make up modern presidential campaigns. All the loser is required to do, really, is have the good manners to congratulate the winner on election night, and then recede to whatever position they desire.
It's like at the end of the Super Bowl, when the losing coach congratulates the winning coach in the middle of the field, but doesn't wait around to stand on the podium as the confetti falls and the Lombardi Trophy is awarded.