The Incas lacked heavy pack animals, so these engineering feats accomplished with only human muscle and intellect are indeed a miraculous accomplishments.  It may be the lack of resources - heavy transport, water, arable land, iron working, etc - conditioned the Incan sociocultural development towards frugality, conservation, and regional specialization.  Abundance of a resource tend to generate intellectual sloth in man, when interacting with his environment regarding the said resource.  

A sociocultural matrix developed from perceived limitless resources, such as the US consumerist social make-up, in which _quantity_, regardless of quality, is prioritized and valued, seems to be a sociocultural matrix quite opposite that of the resource poor Incas.  The vast dust bowls that once dotted the US agricultural landscape would never have been even a possibility for the Incan civilization bred to perceive their environment with scarcity mindset.