Well, in fact, they did! You can read about a selection of North American cities in this post. The majority of native cultures did practice agriculture, and the largest cities, such as Cahokia, grew up to 50,000 people. There was also extensive trade, with links across both American continents. You can find some links and discussion about trade here.

Just a slight edit, maize was actually domesticated from a a plant native to Mexico, not the Mississippi region, 10000 years BP and then dispersed north and south.

As /u/CommodoreCoCo mentioned (and as you can learn about in my post that was linked in his comment), almost all of the Eastern Woodlands (basically the land bordered by the Mississippi River, the Atlantic Ocean, the Great Lakes / St. Lawrence River, and the Gulf of Mexico, with a little spillover here and there) were sedentary agriculturalists. On the Great Plains, there were also sedentary agriculturalist societies and while they were tightly concentrated along the rivers, they would have made up the majority of the population on the Plains at that time. Finally, there were sedentary agriculturalists along the rivers in the American Southwest as well, particular in what's now New Mexico and Arizona. There were also sedentary societies along the Pacific Northwest coast, though these were based on maritime resources rather than agriculture.

There were mobile societies as well, usually on the fringes of the major population centers, living in areas were the climate was too dry for agriculture or the growing season too short, or other food resources were abundant enough to make agriculture unnecessary. However, after the arrival of Europeans and more importantly their horses, many mobile societies rose to prominence, creating the modern impression that such societies were always dominant in North America. Several of these prominent mobile societies arose from previously sedentary ones, for whom the horse gave them ability to pursue the bison herds year-round rather than seasonal hunts that supplemented their agricultural systems.