They did not have animals in north America suitable for domestication to magnify their work potential. They did not have farming technology on an industrial scale, and they didn't have the social structure suitable for long term growth in most cultures. There were a few exceptions in prehistory but they did not survive to contact with Europeans. The Aztecs being the only empire that did exist then. There were a long string of prehistoric empires in the Americas but for the most part they were separated by time and distance from each other.

Edit: the Incas also had contact with Europeans.