If by civilized they mean a powerful imperialist power, which honestly is typically what that word is used to mean tbh, then its because the major imperialist powers pre WW2 had their infrastructure obliterated giving America, which was mostly untouched, free reign to form its empire. It also has very good strategic positioning in that it has two large sea borders which are not only seperated from the "old world" by large oceans but also both coasts are not easily accessable by the other. You'd have to go through the drake passage or the strait of Magellan which are both dangerous voyages in order to access both coasts. Later the Panama canal is built but thats US controlled so good luck with that either. This makes invasions quite hard. Further, the monroe doctrine allowed them to extract some serious resources from latin america while also giving them political power in these regions meaning they gained incredible wealth and secured their southern border. The northern border was obivously secure due to strong allyship.

So the reason it is so powerful while being so young is essentially timing, positioning, and luck.