Humans create images of places which they then come to believe are correct and true.
The Southwest is a cultural landscape that only exists in the minds of humans. Because the boundaries of the Southwest are hard to define, people talk about it in terms of images that seem to most clearly define it. It is not actually bounded by some great wall across which there is another place that is really different. Instead, the place called the Southwest merges imperceptibly geologically and climatologically with places around it. Go find the boundary with the Great Basin sometime, and see if you can actually tell you are there.