> Middle of North America was largely unsettled by Europeans for 350 years.

Easy. Why would you settle there when a much better Eastern coast is available? The US non-Native population in 1800 was less than 6 million.

For comparison, the UK was 10 million according to the 1801 census.

I mean sure. Or you can look up a really interesting feature of American history that is not widely known. The Comanche Empire commanded an incredible amount of space with only 10s of thousands of people.

It's pretty widely known. But what does it have to do with the question from the grandposter?