Why do you think most native american groups did not form large empires like the aztec and inca did?

Most Native American groups could not form large Empires like the Aztec and the Inca did because they were spread out in large regions or territories. This did not allow them to form closed groups and create a large Empire. Also the native groups were of the opinion that the lands could support only a few people and so they never tries to attach with other native groups to create a large enough Empire. Most people lived their life on irrigation and so they did not want more competition from other tribes regarding the land they had.