The problem is this.
These huge areas of terraforming and architectural phenomena are so huge, it makes the great pyramids look like Lincoln Logs.
People marvel at the Nazca images, but that is not even a drop in the bucket compared to all these excavations.
No language? No culture? No cities? No ceramics?
What the heck is this all about?
When the Spanish arrived, most of these terraces were either in use or very recently abandoned, as the first wave of the plagues had swept into S. America only a year or two earlier.
I don’t see anything particularly mysterious about a population of tens of millions in not particularly hospitable terrain creating massive structures to help them feed an expanding population.
I’m not sure what your last paragraph means. S. America is loaded with unexcavated cities, with ceramics, etc. That few Americans are familiar with them doesn’t mean they don’t exist.
“Small strokes fell great oaks.”
I think you underestimate what can be accomplished via persitance and time.
With generation after generation of farmers in the area working on these fields, they could easily have built these structures over time (say over a thousand year period).
To find that stuff you have to look for it. Until recently academic archaeologists were so stuck on "Clovis first" they simply disregarded work showing there were earlier cultures. It's only recently they've begun to accept the documented 15,000-year-old artifacts at Monte Verde in Chile.
As Blam says, there's a treasure trove in South America for a new generation of open-minded archaeologists/anthropologists who follow true scientific procedures. Consider all the evidence of not only advanced agriculture but sophisticated knowledge of soil chemistry in the jungles of Brazil. The search has just begun.