Capitalists have claimed responsibility for America's past economic success. Let's begin by setting the record straight. American success had little to do with capitalism. This is not to say that the US would have had more success with something like Soviet central planning.
Prior to 1900 when the frontier was closed, America's success was a multi-century long success based on the plunder of a pristine environment and abundant natural resources. Individuals and companies were capitalized simply by occupying the land and using the resources present.
As the population grew and resources were depleted, the per capita resource endowment declined.
America got a second wind from World War I, which devastated European powers and permitted the emergence of the US as a budding world power. World War II finished off Europe and put economic and financial supremacy in Washington's hands.