It's cliched, but I think a lot of it is due to people's lack of understanding and appreciation for history; people view history as "the past," failing to really think about the fact that every single person throughout history lived in "their equivalent of 2020" ... in other words, no "ancient" people ever thought of themselves as anything other than the most advanced, tolerant and *post-historical* people yet to walk the Earth. Our basic evolutionary instincts led rise to things like tribes and even nation states in an effort to provide at least those "closest to us" with a better life, often with the absolute necessity that someone loses out. I think there is a balance that can be struck where we might acknowledge that we shouldn't "steal land just because we can" anymore, but once you start putting pressure on entire countries or ethnic groups to apologize for a past they had no part of, it's a really dumb and quite slippery slope. The US could conquer Central America tomorrow, but we don't; and we shouldn't. However, just because our varied European ancestors took land from technologically inferior previous inhabitants doesn't mean we have to go back to the drawing board taking pride in literally anything about our current country. Do we live on "stolen land"? Of course ... honestly, the most that should be done about that is maybe some government-sponsored programs to try to better the lives of Native Americans in our country ... everything else is just a big fat "eye roll."
Many people in the past wouldn't have thought of themselves as the most advanced and tolerant, as these concepts wouldn't have meant much to their cultural self-understanding. But your broader point is right; there's a tendency to think history didn't really happen anywhere until Europeans or their descendants showed up. You can find maps of the indigenous peoples of North America that don't have any date attached, and don't represent any single period in time, but show who was in New England in the 1500s and who was out West in the 1800s on the same map. There were significant migrations of peoples across hundreds, even thousands of miles pre and post Columbus (ex. the ancestors of the Navajo were in northwest Canada one thousand years ago). And ethnogenesis - the creation of a new ethnic identity through splitting and/or combining - was very common. These fact lead to a lot of controversy over who should qualify for federal tribal recognition, as well as the repatriation of ancient human remains and artifacts under NAGPRA. These stem from arguments over whether historical legal documents and archaeological finds, respectively, have any connection to a specific modern American Indian people. Devout Centrist is right that treaty obligations are important, and we ought to honor them as much as we can. But history makes this complicated.
