I think this map is fascinating.
Why so many counties in the eastern U.S. and so few out west? The answer is purely historical. Eastern counties were founded before there were railroads and automobiles. County governments, which handled essential services such as courts, tax collection, and law enforcement, required smaller counties because most people traveled by foot or on horseback. As populations grew, eastern counties were repeatedly divided into smaller ones to keep government accessible. The county I live in, Mecklenburg County, VA, was created on March 1, 1765, having split off from Lunenburg County. Charlotte County was established in the same way.
In contrast, much of the American west was settled later, after railroads, telegraphs, and automobiles made it practical to administer much larger counties (often with far fewer people). For 27 years Becky and I lived in the city of La Mirada, California, in Los Angeles County. Parts of the original Los Angeles County later became San Bernardino County and Orange County. The county seat of Mecklenburg County is the town of Boydton. The Boyd name is a very common one in these parts. In fact, the original owner of my farm was Anderson Boyd. A former member of the 59th Virginia Infantry, he is buried in our farm cemetery.
Final fun fact: Maui County in Hawaii includes the islands of Maui, Molokai, Lanai, and Kahoolawe!