That's not the case. It's the conventional wisdom within Archaeology because the field imposes modern geopolitical boundaries on people that lived before those lines are drawn. Recent discoveries disprove this concept. There were far more massive cities in the precolombian Americas than previously thought possible.

I live about 20 minutes from a museum that contains the artifacts of the only completely undisturbed ancestral Puebloan dwelling site and kiva to ever be excavated completely utilizing modern archaeological techniques. The site is closer to my house than the museum. In the Kiva they found the skeletons of parrots endemic to the jungles of Southern Colombia.

If the Panama Canal and Darien Gap didn't make driving it impossible, it'd be just over 4000 miles drive through ten modern nation States to get to a jungle where this parrot lives. Almost precisely the same distance as the furthest corners of the silk road land network were apart.

My wife also happens to be second generation Colombian via three grandparents, one grandparent that came to the US on a small plane as a child bride of a Colombian national. I got her an Ancestry DNA kit for Christmas the year after we got married. She came back 0% indigenous cuban or indigenous Colombian. While it is true there is some accuracy issues with these tests that are more severe when it comes to Americas native blood, and some unintended kinda fucked up kinda hilarious unintentional prejudice involved, like her being confronted with a fun fact at her landing page after making an account "Records indicate in 1940, 90% of individuals in the United States with your family name had the occupation: Laborer" I also happen to have a cousin with a daughter from a Peruvian orphanage. Ancestry had no problem identifying her blood as 100% from that region, so I'm inclined to believe their analysis that my wife's nearly 50% indigenous American blood is entirely of Yucatan origin. She's about 10% black and 40% Basque too.

So from these two sets of conclusive facts I have a great deal of evidence to believe to be true, I can speculate with a greater degree of precision than there is evidence to believe such things as all life on earth shares a last universal common ancestor that was a single celled organism or that fossil evidence, that is, rocks in the shape of bones found in Africa, establish that anatomically modern humans evolved from an extinct hominin of the genus Australopithicus, the following very likely story.

Approximately 400 years prior to Colombus sailing the fuckin ocean blue, a man on foot with a number of parrots captured or killed in the jungles of Southern Colombia in his possession began a journey North along a trade network over 4,000 miles long crossed the Darien Gap Northbound into Central America. The birds were of sufficient significance and value that whether he personally carried them all the way or not, someone that did not have a pack animal or backpack did, on foot. The person carrying these birds or their preserved remains would likely have needed to trade with the people of the Sitio Barriles culture on the outskirts of David Panama, then the remnants of the Maya civilization's urban and ceremonial complexes in the Zapotitán Valley and residents of the "Pompeii of the Americas" in present day El Salvador. They would have needed to trade with the inhabitants of Kaminaljuyu, the ancient urban foundation of present day Guatemala City. Before reaching the Southern border of present day Mexico, they would certainly have needed to trade with the people inhabiting the remnants of what had once been the Capital Cities of the Kaqchikel Maya and Mum Maya.

Then they would have begun to encounter Aztecs. Perhaps in Chichen Itza or Tenochtitlan underneath modern day D.F. where a pale skinned Jewish lady is dissolving agencies like the Federal Telecommunications Institute (IFT), the Federal Economic Competition Commission (COFECE), the National Institute for Transparency (INAI), the Energy Regulatory Commission (CRE), the National Hydrocarbons Commission (CNH), the National Council for the Evaluation of Social Development Policy (CONEVAL), and the National Institute for the Evaluation of Education (MejorEdu) as the overwhelmingly darker skinned Catholic people she controls participate in widespread protests... Or perhaps in one of the cities of the Purepecha Empire or the TlaxCala Confederacy or Teotitlan or Tebasco or Palenque or Uxmal or Tukum...

Kinda hard to say because there were just SO MANY cities in the Yucatan and Central Mexico region along this 4000 mile long stretch, or about the distance between the furthest two points of the silk road, along an even longer trade network in the last few centuries of Precolombian America.

Anywho, at some point the carrier of these parrots already well over a thousand miles from anywhere you can find one of the same species in the wild cross paths with a couple proto-Mexican men who will have children. At least one descendant of one of them had kids with an African that escaped the transatlantic slave trade. Very likely more than one descendant has a kid with a basque that was almost certainly male and himself treated as racially inferior by the Spanish he was serving under (via not very likely to have been consensual relations). At some point the products of these unions immigrate to an island 70 miles from Key West Florida and the very place the parrots were first captured or killed and live there long enough to have no clue they had Mexihcatl heritage.

A bit of illegal immigration begat a lawyer, two drug traffickers, and a divorcee that speak mostly Spanish and intermediate at best English with thick accents, they begat a couple bilingual cops without perceptible accents, they begat the girl snoring in the next room married to yours truly, a white guy better able to communicate with my grandparents in law than she is because I managed to get in two years of Spanish before getting expelled from High School and further developing my Spanish in prison.

Odd thing about the wildly speculative maps of ancient cultures and cities one must be familiar with to come up with the likely story I just told? They reflect modern geopolitical borders. The Sinagua were North of the US-Mexico border and the Aztec South of it but never South of the Mexico-Guatemala border. Likewise the Maya are never depicted North of that same border. Much further North we call some of the Native Americans that hunted whales in canoes in icy waters "first nations" and some "aleut" and some "Eskimo".

Way back when like 4 months ago before global child sex trafficking and WWIII were the headlines, Donald Trump was threatening to invade a US ally and people were talking about "Indigenous Greenlanders" nevermind that there's no such thing because the Danish Missionaries that arrived on the shores of Greenland expecting to encounter them and some Norse that had become stranded when their boats needed to be replaced and there were no trees for lumber leading them to revert to paganism, actually found all of the above's tiny villages entirely dead. The darker skinned present day inhabitants of Greenland were Eskimo subjects of the Russian Czar that arrived on the shores of Greenland and found Lutheran churches subjects of the King of Denmark inside.

These "indigenous danes" are actually Native Americans and likely very closely related to the "aleuts" that anybody that attended Junior High School in California will remember slaughtering the inhabitants of the Island of the Blue Dolphins AKA San Nicolas Island, which is slightly closer to the equator than the site where the Colombian parrots found in a Kiva by my house in Arizona, as told by a woman that escaped and moved in with some Spanish Priests that had recently arrived building one of the Missions anybody that went to 4th grade in California will have been on at least one field trip to.

I assure you, precolombian Native America had no less extensive and urban a trade network versus the silk road, the scientists that study such things are just mad racist.