Because the First Lady does not get paid.

While the position carries no official duties, the president's spouse has long been expected to serve as a highly visible goodwill ambassador for the nation, performing a wide range of ceremonial and quasi-diplomatic jobs, reports Politico.

The work involved is not insubstantial. Because they are presidential spouses, First Ladies are expected to volunteer their assistance.

So from January 20, Michelle Obama will go from being a high-profile, highly compensated professional to serving as her husband's full-time, unpaid ... helpmeet?

Resuming her previous work for the University of Chicago Hospitals would be difficult in her husband's conflict-averse regime. But giving up her paycheck and re-envisioning herself in the role of hostess in chief will undoubtedly be an adjustment for the Harvard Law grad.

Yet as anyone who has ever been to a cocktail party knows, being gracious to strangers requires both effort and art, and when the First Lady hosts dinners, receives visitors or occupies the wives of foreign leaders, she is performing vital if unsung services for the administration - services that would otherwise have to be performed by salaried staff.

The post isn't all tea and small talk, either. As the travel demands on the president have grown, the First Lady has increasingly become a stand-in for her spouse.

Without Barack Obama's books, the Obamas' 2006 tax returns show that Michelle Obama would have been the hands-down breadwinner in the household, bringing home 316,000 dollars, or nearly double Barack Obama's Senate salary of 165,000 dollars.