Why most world deserts are located in the western margins of the continents in the subtropics?
While some deserts are on western margins of continents, most desert land isn’t. The main Asian deserts happen to be in the middle of the continent, the Sahara spreads right across North Africa from west to east, and Australia’s deserts are also largely central. And then there’s the entire continent of Antarctica, a desert due to its low level of precipitation. Details like that mean there’s something wrong with the assumption behind your question.