Why is the western United States a desert and there are so many people?

Because of the early "westward movement" and the water source.

/kloc-From the end of the 8th century to the beginning of the 20th century, the United States launched the "Westward Movement" against the barren western region, which attracted a large number of people to move westward and developed the western region, forming many cities, among which Las Vegas, Phoenix and El Paso, which are located on the Mexican border, are the larger cities.

Almost all the land in the United States has rivers flowing through it (areas without rivers are not too far away from the rivers, which is convenient for water diversion). There are many high-altitude mountains in the central and western regions, such as the Rocky Mountains, the California Mountains, and the highlands of Arizona, which provide water for rivers.

for instance

Take Las Vegas as an example. Before 1930s, Las Vegas was a small village, but with the completion of the Hoover Dam on the Colorado River, water was transported to the city through aqueduct, which solved the water source of Las Vegas, and the population quickly gathered, forming a city on the Gobi Desert.

Similarly, there are Gila River and Salter River near Phoenix, from which water can be taken as the fundamental driving force for urban development.