What areas and cultures in North America are you referring to specifically? The Mississippian cultures of the American Southeast were extremely complex states and not nomadic in the least. The Ancestral Puebloans of the Southwest were similarly sedentary and engaged in a transcontinental trade network spanning from coast to coast and from the highlands of Mesoamerica to the great plains of North America. The tribes of the Pacific Northwest were culturally distant from either of these groups, yet they maintained a complex stratified society on the abundance of local resources.

Nomadic tribes were present all over the Americas. In some cases, groups would switch from sedentary to nomadic lifestyles or vice versa, as happened with the Mexica and the Navajo. By no means is there a clear delineation between "North American nomadic cultures" and "South American civilizations". Some nomadic groups were responding to local environments, as the Western Papaguerians responded to the inability of the Sierra Pinacate to support agriculture by hunting or Plains tribes by following the Bison. In some cases, groups were responding to the political realities of their area, as the Chiricahua Apache did under Spanish rule.

Also, there isn't a sharp delineation between "North American nomads" and "South American civilization". The states in the American Southwest shared decidedly more culture with Mesoamerican groups than they did with the plains tribes, even if the latter were geographically "North American". Secondly, South America had perhaps more nomadic groups than North America did. The Amazon basin was absolutely filled with them and South America as a whole held more people than anywhere else in the Americas.