I agree this is tragic and under-reported, but I think you weaken your case when you exaggerate like this.. Its not a direct contest for "greatness" since they were so different.
It must have been truly something to behold even by today's standards.
As the other response notes, Tenochtitlan was possibly the largest city in the world at the time, and the Aztec empire contained many other cities with populations over 10k. The Inca built a massive empire (8-12M people) centered on the Andean mountains linked together by roads and suspension bridges and built monumental architecture using masonry techniques that are impressive even today. We're just starting to get a handle on the scale of the Mayan empire, because they built their cities in parts of the Guatemalan jungle we have a hard time getting through today - some of the more recent work has found evidence of raised causeway networks stretching over a hundred miles linking large scale settlements. Go further south, and the Amazon jungle basin has been considered uninhabitable because of the density of the jungle and the poor quality of the soil, except that we've found evidence of cultivation of plants and more recent evidence of large-scale settlements and waterworks.
The Spaniards did not walk into a backwards people or somehow just miss what they were looking at - the civilizations and cities they found were advanced and obviously developed and on a scale that, even outside Tenochtitlan, would have rivaled European cities at the time for both size and sophistication.
the story of Jonah in the Hebrew bible describes a city of 100,000+ people .. and that was just one city somewhere at that time.. that was three thousand years ago in the Middle East
Cities were effectively constrained by available food and transportation costs before the steam engine, so you'd get cycles of population growth, followed by population decline.
Particular places reached support capacity of surrounding land until something caused systemic failure, and population decline. Think of ancient China, for example with its cyclic population growth in a unified empire, followed by some natural catastrophe or political instability -> civil war as the political system weakened.
And they also ritualistically tortured, killed and cannibalized children during the dry season. https://www.eiu.edu/historia/Thoele.pdf
That was extreme and insane to most of the native inhabitants. And it was even more outrageous to Europeans (or Asians). The Spanish weren't hero's and they exploited the natives and unwittingly spread disease but the Aztec civilization had to go and it died just as much from internal revolt than from the small amounts of European men that Cortes commanded. I have more respect for the Pueblo's than I do for the Aztecs. They survived harsher environments and had impressive self-taught farming and structural engineering skills.
The book "Why Nations Fail" spends a while discussing the Spanish approach to the Americas, and "Extractive" doesn't even begin to cover it.
https://en.wikipedia.org/wiki/Human_sacrifice_in_Aztec_cultu...
The conquistadors came across full-fledged empires with sophisticated arts and cultures who’d built cities that dwarfed their European counterparts with building techniques and on terrain that would’ve flummoxed western builders at the time (and still cause us to pause today) and destroyed as much of it as they could put their hands on. Smallpox was the disease, but the Spaniards were the plague.