North America did have at least one large urban civilisation- the Cahokians. They built large urban complexes in the Mississippi Valley until the collapse of their society, probably from the waves of disease racing out from Columbian contact. The northeastern tribes, while not at the same level of urban development were certainly not hunter gatherers either- the first records post contact indicate thriving densely populated villages in a cultivated landscape. Again it's the collapse brought about by disease that makes these groups seem much less sophisticated than they were, because you're essentially looking at a post apocalyptic society.

There's always room for discussion, but perhaps the section Technology and Civilization in the Americas from our FAQ will answer your inquiry.