Empty land because its rainfall is insufficient to grow traditional crops. Worse, the water table in much of it is quite deep and the groundwater opportunities are scarce (and becoming more so). Compare with the US east coast where its pretty much impossible to throw a rock these days an not hit someone's house. Even in the rural areas people are frequently on plots of land that wouldn't support them without intensive industrialized farming.

Check out the "field lab" video's for a now ten year video blog posting of the hardships of attempting to live off the land in the arid SW us. Somewhere along the way he started saying "i'm just not someone who can grow crops". But in reality, its more a case of he can't grow crops because they don't survive 100F summers, or seasons without a lot of rainfal. If the weeds are having a hard time growing, your potato's/whatever are probably not going to be doing well.

edit: just to point out the redundancy, if you go read the newspapers in any of the major cities in the SW what you will frequently read about is water woes. Overwhelmingly those cities ability to supply water at the projected population growth rates are questionable.

Yeah CGP Grey had a god video about federal land ownership the majorty of land in western states is federal land.

https://youtu.be/LruaD7XhQ50

Its kind of ridiculous. how little control states have over the land within their own boarders.