Not sure why I can't reply to the sibling comment...
anyway, the reason for the east/west divide in population density is that the west is dry. See maps at climate.gov (https://www.climate.gov/news-features/featured-images/new-ma...). By convention people refer to this line as being at the "100th meridian" but the 98th may be more accurate.
Bit of a tangent, but I've always wondered why the United States has a very clear dividing line between the populated east and relatively empty west. It's really visible in the first map you shared. Is there a historical reason behind that line? Or some geographical boundary?
My understanding, as an american, is that the Federal government "owns" most of the land west of either the Mississippi River or the Sabine. Or used to "own" it. So getting land for personal/business use in that geographic region is harder than eastward. I don't know the term of art, but it's like the Federal Government - via the BLM, USDA, USFS, or whatever - has right of first refusal for all land sales.
I know that my understanding can be wrong, but this is what i've heard and perusing some maps bears out at least some validity.
The more interesting map is the one where you weight the display by population, since it gives a much more accurate representation of how the USA is divided: https://engaging-data.com/pages/scripts/d3Electoral/countyel...
The tool on that website is actually pretty informative to play with if you want to quickly see how adjusting the map style can change the perception. https://engaging-data.com/county-electoral-map-land-vs-popul...