So here is a map of the valley of Mexico, before Lake Texcoco was drained. As you can see, there are flat areas, especially along the lakeshore, but the valley is lined with hills and there are even hills between major population centers. This isn't to say there are no flat areas, but elevation is a major concern for any significant travel distance.

Here is a map of the Incan Empire. Notice the terrain between major population centers like Cuzco and La Paz or Lima? Again, there are places where a wheeled cart would have helped, but overall the land is quite mountainous.