The size of the Sahara Desert is often compared to the US, so maybe that's why.
Also, on this map, the southwest US overlaps a very densely populated and well-watered part of Africa.
Can you elaborate on Africa's inhospitality?
I'm sorry, I didn't mean it that way. I meant the Sahara is the least habitable part of Africa, and also in the top couple least habitable places in the world.