The lower map on this site shows the population density of the continental U.S., broken down into counties. There appears to be a change in density about half way across, beginning with the Red River, separating North Dakota from Minnesota, and continuing south, into southern Texas. It looks almost as if there were a natural barrier there, such as the beginning of a mountain range or desert. Except that there’s nothing there; there’s nothing that explains a sudden change in population, separating the eastern half of the country from the western.
People used to believe that the middle of the continent was a desert and unsuitable for farming. So rather than settle there they traveled through it as fast as possible and settled on the Pacific coast.
The northern end makes sense. You start running into some bad country in the Dakotas (called Badlands, if I recall correctly).
Further south, it looks like population dips past the last big rivers (the Missouri and Arkansas mainly). That makes sense. You need both water and waterways to support a population. The land past there is mostly dry, flat prairie without too many major rivers. Where you get population density picking up again tends to be near water in Denver and Santa Fe. Montana is a bit of an exception, but there’s some tough ground to cover before getting there from the East.
Further south in Texas, the piney forests and hill country end in the middle and you again hit the dry, relatively flat land that’s the west part of Texas.
Relatively speaking, it is desert. If you want to grow crops in the western half of the Dakotas, Nebraska, Kansas, or Oklahoma, you need artificial irrigation, something that wasn’t easy to do on a huge scale until relatively recently. Likewise with establishing a village/town/city 150+ years ago: you needed a river or a lake, something that can provide a lot of water to meet the needs of a growing metropolis.
Those things are less of a constraint now that we drill ridiculously deep wells and pipe water across miles and miles of countryside, but it might help explain why the settlement of these areas is far behind other parts of the country.
The Mississippi River is a nearly straight line running north-south as are the Rockies. What we’re seeing is a reflection of underlying river systems and geology.
The population density of Egypt does much the same thing. It follows along the Nile and drops off fast past it. Ditto most major river systems in Europe and Asia. They mostly don’t happen to run straight north-south.
The homestead act of 1862 gave 160 acres of land to the settler who lived on the land for five years and improved it. According to some historians, that was only enough land to support a family east of the 100th meridian, but not west of it where the climate was too arid.
