I think it's because deserts grow. All that dry hot wind blowing sand around tends to increase the size and ruin "normal" land and without any large mountains to stop the clouds they just blow right over or get evaporated by the heat...it's like a giant mirror reflecting light and heat. Millions of years ago, the Sahara was a swamp, but now it gets bigger every year. I'm sure there's some scientific stuff behind it but this is just what I remember from my schooling.