There are large sections of the US that are mostly uninhabited, but they don't have to be. People can live perfectly well in Kansas and Alabama, for instance. No one has to truck in water, like they do to southern California, to support the population. But for whatever reason, very few people live there.
On the other hand, there are also plenty of places where there's good reason no one lives there. I think we've driven through a lot of them this summer. You can see fresh volcanic action from Yellowstone all the way to the Nevada/California border, for example. Hard to plant crops on straight obsidian, you know? These places are dry and rocky and completely alone. Take Craters of the Moon, for example. If you drive across southern Idaho on the 20, you'll go right over the top of Craters of the Moon, and see some seriously weird landscape. It's part of the Oregon Trail, and I have no idea how they managed to pull wagons over this. The ground is covered with bottomless cracks and glass-sharp volcanic rock and lumps and ropes of hardened lava. It's like being on the moon.
Drive a little further, closer to Mountain Home, Idaho, and you find these strange rocks coming out of the hills all over. It reminds me of Nemrut Dagi in Turkey.
Eastern Oregon is an interesting place. (Kind of uh, subtle, too.) You hear the word Oregon and you get excited about rain and jungle and green. But eastern Oregon is NOTHING like that. It's a vast expanse of dried grass and low hills, and there are very, very few people. Mostly the only settlements are around occasional rivers or creeks, and those places are irrigated. But the rest is dry.
If you want to go from Boise to say, Sacramento, you have to go through Nevada. The northern end has dry grass, as opposed to the Vegas area, which is mountains of dead, dry rock. If you love the desert, this state is for you! Northern Nevada has the population metropolis of Winnemucca:
When you get out of Winnemucca and onto I-80, you'll see tons of white sand and salt at the sides of the road. People have picked up blackened rocks all the way across 80 in Nevada (and through the salt flats in Utah as well) and drawn pictures or printed words for your viewing entertainment. They are usually on flat ground, so it's hard to get a good picture. This was my best shot:
Then there's the central valley of California. It looks like this:
Again, just blank, faceless yellowed grass for miles upon miles upon miles--but if you just add water, you get enough acres of crops to feed the nation.