Cities in California have green areas thanks to irrigation.
Once you leave them, however, the view on both sides is flat
scrub leading up to distant mountains. Trees like this are only
found near water sources. Once you're into Arizona, the lush
landscaping typical of the California oases gives way to miles
upon miles of desert.
This is a more typical view and characterizes much of the unpopulated
western United States. The main difference is the size and distribution
of the sagebrush.
The drier it is, the smaller and farther apart the bushes are.
Some stretches are so dry, they are mostly sand and rock.
The Phoenix area is full of these rocky hills.