One way to compare the real purchasing power of market wages in different countries is to determine how long it takes to be able to buy an item that is available in most places in the world. One such product is McDonald’s Big Mac sandwich, which is sold in the company’s restaurants around the world. In the United States, a typical worker spends about nine minutes on the job before earning enough to buy a Big Mac. A Nigerian worker who earns the average market wage must work an hour to earn enough to buy a Big Mac. In Kenya, the time necessary to earn enough is about three hours. Thus, the purchasing power of the average market wage in the United States is considerably higher than in Nigeria and Kenya. 

What would explain the purchasing power of an average wage being so much higher in the United States than in Nigeria and Kenya?