They are also loaded with people. America was a free-for-all (except the American Indians, of course, who were overpowered by superior technology).

Maybe there was a lot of wealth in silver, I don't know about that. It seems that it had some land-based oil that they started extracting in the beginning of the 20th century, but I've no idea if they were as extensive as in the US (where there was Standard Oil, an immensely wealthy corporation).

Mind you, I'm not saying it was only the natural resources, there are other factors as well - different climate, geography, cultural background (English vs. Spanish/Portuguese), colonial history, ... I somewhere read that it's a curious phenomenon how so many countries in SA had so many dictatorships in sequence...