Those who credit american ingenuity and exceptionalism must acknowledge land is by far the most precious commodity, an entire continent and a beautiful and rich one like North America is essentially priceless, and will enrich generations to come till the end of time. Much much more than $90 or $900 trillion. The economic value of this immense tract of free land and slave trade cannot be brushed aside.
But there is also credit to the courage and ingenuity of the early immigrants who took perilous journeys leaving everything behind, risking diseases and an unknown and uncertain future, and made something out of it.
One would like to think those who are proud are proud of this and not the usurpation of a continent or slave trade.
You have to consider geographical advantages when considering the growth of a civilization. For instance, the Mississippi River has amazing properties for transportation which was obviously great for trade, manufacturing, and settling. Many rivers in Africa may seem similar on the surface but due to things like frequent waterfalls and variation of depth, they are not nearly as advantageous.
This stratfor article discusses the natural capital afforded to the US:
https://www.stratfor.com/analysis/geopolitics-united-states-... both in terms of transportation networks and being removed by an ocean from the costly power struggles of Europe (which I'll add can be seen in the numbers of casualties for Americans in the first and second world war relative to our European counterparts and realized gains from those wars).
There is also an interesting observation by Nick Szabo about the value of a transportation network: '
Combine this with Metcalfe's Law and we reach a dramatic but solid mathematical conclusion: the potential value of a land transportation network is the inverse fourth power of the cost of that transportation. A reduction in transportation costs in a trade network by a factor of two increases the potential value of that network by a factor of sixteen.'
But there is also credit to the courage and ingenuity of the early immigrants who took perilous journeys leaving everything behind, risking diseases and an unknown and uncertain future, and made something out of it.
One would like to think those who are proud are proud of this and not the usurpation of a continent or slave trade.