Extensive deserts occur in the Western Tropical Regions of Continents because

Extensive deserts occur in the western tropical regions of continents due to the combined influence of offshore trade winds and cold ocean currents. In these latitudes (15°-30° N/S), the easterly trade winds blow from east to west; by the time they reach the western margins, they have already shed their moisture on the eastern coasts, becoming dry and offshore [c2, c3, t1]. Additionally, these western coasts are bathed by cold ocean currents (such as the Humboldt or Benguela), which create a 'desiccating effect' [c1, c2]. These cold currents chill the air, leading to temperature inversions that inhibit convection and stabilize the atmosphere, preventing precipitation despite high humidity or fog [c1, t1, t9]. This synergy of dry, subsiding air from the subtropical high-pressure belt and the cooling effect of coastal currents results in extreme aridity [t1, t3].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 32: Ocean Movements Ocean Currents And Tides > Desert Formation > p. 496
2. [2] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 7: Arid or Desert Landforms > Types of Desert > p. 67
3. [3] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 18: The Hot Desert and Mid-Latitude Desert Climate > Distribution > p. 173
4. [4] https://education.nationalgeographic.org/resource/desert/