This is part of the mythology of the American frontier. Just as America itself was a land of untapped potential for European settlers who had enough of the decadent, stifling old countries of Europe, the West was a land of untapped potential for Americans who had enough of the decadent, stifling East Coast. Just as unpopular New religious sects left for America, the Mormons left for Utah. The gold and silver rushes were outlets for an ambitious American Dream. The verdant fields and valleys of Oregon and California were outlets for ambitions of their own.

The US ran out of frontier around the turn of the 20th century. And this is problematic, because no populations is going to stay as risk-tolerant and ambitious as the population that already risked their lives to travel there in the first place. Immigration might actually refresh these attitudes, but you’d expect them to erode over time. Perhaps they have.