First, an important clarification: there is, and never has been, a single “Indian civilization”. The description you provide fits primarily with Plains cultures, and even among these cultures sharing similar characteristics, there were numerous different groups - from Comanche, to Lakota, or Siksika, and numerous other Plains people who hunted bison, lived a nomadic lifestyle, and used the tipi as one of their primary dwellings. For a number of reasons Plains peoples have become the best-known and most stereotypical depictions of Native Americans, and overwhelmingly feature in popular media.

To clarify, peoples like this absolutely did exist prior to Columbus. Bison hunting is a tradition stretching back thousands of years in North America, and nomadic hunter-gatherers were absolutely present on the continent in 1492. Of course, there were differences - without horses, for instance, dogs were used to transport goods. But these groups were far from the norm, and I should stress that they have never been the norm across the continent.

The Great Plains are historically the heartland of Plains cultures, and this is the region where groups like the Lakota and Comanche rose to prominence. But in 1492, huge swaths of the Plains were occupied not by nomads, but by farmers living in settled villages along river valleys. These were the ancestors of today’s Wichita, Mandan, Hidatsa, Arikara, and others; the Wichita dominated the upper Arkansas River in Kansas and Oklahoma, with early Spanish accounts describing villages reaching up to 20,000 people. The Mandan, Hidatsa, and Arikara, further north in what is today Nebraska and South Dakota, had fortified villages with up to 2,000 people. All these groups depended on growing maize (corn) and other crops to thrive; though they hunted buffalo, it was a part-time occupation for them. Importantly, this remained true all throughout the colonial period. The Mandan, Hidatsa, and Arikara are still located in North Dakota, where they have been farming the Missouri River since at least the 17th century. The Wichita are also still around, in Oklahoma. None of these groups ever abandoned agriculture. They have been farming the Great Plains continuously since long before Columbus was born.

In fact, Great Plains agriculture was not even at its peak in 1492. That had come earlier, when farming cultures inhabited regions as desolate as the Texas Panhandle. They had abandoned the region after protracted droughts in the 15th century forced them out.

Of course, this is only the Great Plains. The North American continent is much bigger than that. In the Pacific Northwest, you had complex, hierarchical fishing-hunting-gathering societies living in permanent villages along the coastline. In California, numerous bands of hunter-gatherers subsisted off the fertile landscape. In the arid southwest, the region’s oldest farmers lived in adobe towns with up to thousands of residents, farming maize and beans and raising domestic turkeys in the region’s river valleys. In the southeast, the moundbuilding Mississippian cultures lived in agricultural villages and towns, with powerful rulers in the walled mound centers exercising great power over what were sometimes quite large domains (the paramount chiefdom of Coosa, for instance, ruled much of northwest Georgia, southeast Tennessee, and northeast Alabama; the paramount chiefdom of Cofitachequi may have ruled over most of what is today South Carolina). In the northeast, large agricultural towns of several thousands dominated the landscape, with a complex network of alliances leading to the formation of groups like the Haudenosaunee (Iroquois) Confederacy.

All of this is to say that North America in 1492 was a complex, dynamic place, with an incredible diversity of cultures and lifestyles. So yes, the continent was not the Hollywood version of Plains cultures. And indigenous societies absolutely did, as you asked, have proper towns and organized political and physical infrastructure. Most of the continent east of the Rocky Mountains were settled town-dwelling agriculturalists, not nomads. But on to the next part of your question: what happened?

Because there were cultures that abandoned agriculture, took up the horse, and became nomads. We can see this with the Cheyenne and Lakota. But most peoples didn’t. Native Americans were, and remain, incredible agriculturalists. When the first English colonists landed, they were greeted by cultures growing maize in abundance. And they never stopped. All throughout the northeast, midwest, southeast, and southwest, indigenous groups retained significant elements of their precolonial lifeways. They are still here, and still growing maize, beans, and squash like their ancestors - even if the techniques have changed a bit.

The theory that a massive epidemic sweeping its way across the continent killed up to 90% of Native Americans before any Europeans has been floated in the past, but it’s not widely held today. It doesn’t hold up to archaeological or historical scrutiny. Many indigenous groups grew in population throughout the 16th and 17th centuries, and only started declining precipitously once Europeans started settling the land, killing, enslaving, or forcing out the Natives in the process. So no, there probably wasn’t any unseen apocalypse, even if some disease did arrive before colonizers did.

So why did the Cheyenne and other peoples abandon agriculture to take up horseback buffalo hunting? I think there’s a degree of modern chauvinism in assuming it must have been an apocalyptic event to lead to such a societal shift. An agricultural society is not inherently better than a non-agricultural one, and abandoning agriculture is not inherently a “regression”. More likely, these peoples did what they viewed as best for their society - and they certainly did well for themselves on horseback, so they weren’t exactly wrong.

I noticed my question started to attract some downvotes - how could I improve it so it fits the sub more?