America was a genuinely high-growth 'emerging economy' until ~1950, and then by sheer coincidence experienced another few decades of economic dominance thanks to the destruction of WW2 and the rise of communism in eastern Europe and Asia. The 'glory days' were quite real, at least economically. As for other claims, it's easy to feel optimistic when a rising tide is floating all boats.
The USA was blessed with some great geography too. Two giant mountain ranges capture enormous amount of snowfall and the snowmelt drains into the interior of the country, rendering massive areas amenable to agriculture. In turn this created a large population suitable for industrialisation.
