Complex society -- legislatures, economics, the works, even writing -- could easily have occurred hundreds of times over the millennia, without building with stone and so leaving no trace after they collapsed or were wiped out by invaders. We know about a precious few that did build with stone, that could have been as socially sophisticated as you like, but left no trace of that.

We know extremely little about people in the Amazon basin, beyond that they had already domesticated orchard trees by 10,000 years ago, built over wide areas, and had no access to stone. They had ceramics, but did not favor bricks. They left us a few enigmatic inventions including curare and terra preta.

Assuming primitive social relations is very close to racism. Sophistication seems to need nothing more than population.

Population demographics are one of the things archaeologists are most interested in. There's lots of independent methodologies that we've developed to tell us when people were present and in what numbers. Many don't even rely on finding artifacts. Things like genetic work directly tell us effective population sizes at various points in human history (low), charcoal analysis tells us how many fires were burning, hearth counts (indicated by rock or soil discoloration) tell us how many people were cooking, pollen analysis tells us what the plant landscape looked like in areas, etc. There's even an incredible technique based on eDNA analysis that can directly measure how many people were in the area when a soil sample was deposited by measuring the DNA from their shed cells. I've only seen that one used for results back to 30-40kya, but it's still pretty cool.

As you might have guessed from how I've written this, all these different lines of evidence consistently point to low population densities and overall populations, especially before the main human expansion out of Africa.

Advanced ice age societies aren't impossible, but there would need to be some pretty strong evidence and a good explanation to account for all the data we have against them.

Those tests are run only in places we already knew people were.

Notably, none have been run on the million square miles of Sundaland and the continental shelf off China, now under water, where people lived for at least 70,000 years. We know they invented oceangoing navigation pretty early, because they got to Australia.

People did that another 100,000 years earlier and got to Crete.

The null hypothesis has to be that people in places with sufficient population have complex society, because that is baseline human behavior. You need good evidence of someplace they didn't, and an explanation why not.

The genetic work actually does include those people, namely those whose descendents have been sampled.

So any population wiped out by genocide, famine, or plague is unsampled. The Amazon basin hosted many millions of people for millennia until smallpox, diphtheria, and measles were delivered to them.

No? Even genocides leave descendants. Moreover, who was doing this hypothetical genocide that didn't have kids?

As I said before, it's not logically impossible that a highly populous society existed in the pleistocene. But the constraints it would have to fit grow increasingly contorted as the state of knowledge improves. At some point it starts to resemble a small colony of advanced alien settlers who landed one day and disappeared a short while later. That explanation might honestly be easier to account for than what some people propose, since they don't have to develop their tech from scratch here on Earth.

How strong a signal do you get from the Chatham Islanders, wiped out literally weeks after the Cook expedition told the Maoris about them?

1) the moriori still have living descendents today and

2) it's pretty clear from both this comment and the substantial edits above that this discussion isn't in good faith

Ignoring the gaslighting, how much do we know about the Chatham Islanders' pre-contact culture? And, how many of those descendants have non-Maori Y chromosomes? Any?

    I do not understand why academics think all people on Earth were always at the same technological level at the same time.

They don't. Archaeologists are well aware that terms like "Hunter-Gatherer" and "lithics" subsume huge amounts of diversity covering hundreds of thousands of years, but they've stuck around for legacy reasons. Quoting the intro from my old textbook (The Lifeways of Hunter-Gatherers: The Foraging Spectrum):

    [...] in the models developed in the twentieth century to describe hunter-gatherers, variation was something to be explained away to uncover the essential hunter-gatherer.

There are thousands upon thousands of recognized "industries" of distinct toolmaking traditions and that's only a minuscule proportion of what must have existed.