What does Shames mean when he says, "open space generated not just ambition, but metaphor" (para 12) ?

This is a line in Laurence Shames' "The More Factor." In this essay, Shames argues that Americans have always valued the idea of having "more". He states that this was intrinsic to the development of the nation during the years of the Wild West. People wanted more land, more riches, and were prepared to sacrifice everything in order to obtain it. He also argues that this desire for "more" has meant that Americans have forgotten about other values, concentrating on quantity instead of quality.

Shames also argues that the idea of "open space," of unlimited land that could be grabbed, generated ambition, but it also generated a metaphor. That is, a metaphor of unlimited potential and growth. This metaphor is still associated with the idea of "conquering frontiers." However, he argues that as hopes of growth are now put on the economy, we have to realize that growth is finite, and that we need to reexamine our values and future goals for the country.