The United States grew (colonized, captured, appropriated - feel free to chose the verb that most agrees with your view - it doesn't matter too much) into relat

The United States grew (colonized, captured, appropriated - feel free to chose the verb that most agrees with your view - it doesn't matter too much) into relatively uninhabited land from a starting point with a relatively uniform culture and ideals. As areas got to a large enough population that they could become a state, they were added to the union. South American is already mature. The countries there have different political situations and relationships. Trying to bring them into one federal republic is a non-starter -- it ain't gonna happen. A better model would be the EU where you have mature countries removing trade barriers between them, allowing free immigration, and a single currency. This was anchored by a few large countries that had relatively low corruption, peaceful relations and strong economies. South America... doesn't quite have a strong set of low corruption and strong economies. There are regular coups and a number of ongoing civil wars that would make trying even for a loose coalition of countries in South America difficult.