I always wondered this question for a long time, years, decades.
One day, a guy explained it to me. It is very long-winded and complicated, but basically it boils down to this.
The United States and England setup a system in 1945 called Bretton Woods. Under this sytem, the United States would be the world reserve currency with the pound sterling being like a co-reserve.
This meant the highest valued currency in the world would be the pound sterling and next the dollar.
Since the dollar and pound sterling MUST BE OVERVALUED to be a world reserve currency, all currency in every other country is necessarily undervalued.
In 1945, the only nation with an intact manufacturing sector was the United States. So, the world relied on the good ol' US of A to provide the manufactured goods until they could rebuild their economies.
By 1970, many countries caught up with the United States, which caused manufacturing and jobs to start leaving. To get around this, the United States made sure oil was traded in dollars and pound sterling.
By 2010, the whole world (just about) is more competitive than the United States. Yet, the United States and Great Britain still want to be the world reserve currency to control the oil.
But, the days of cheap imports are numbered, and eventually manufacturing will return as the dollar and pound sterling devalue into oblivion.