Why do many deserts exist at 30 degrees north and south latitudes?

B. these positions are roughly equivalent to the tropics of capricorn and cancer. these points are furthest points on the globe that can point directly at the sun at some point during the year. anything beyond these points can't point directly at the sun (due to the tilt of the earth's axis) and thus don't recieve as much sunlight as the areas between the tropics.