@Zerlyna@Zerlyna because desserts are places with very low precipitation, which. Is correlated with but not solely caused by high temperatures. Due to proximity to the ocean and existing wind patterns much of the tropics experiences high rainfall.
The obvious exception is the Sahara which is landlocked for the most part and no longer favored by the Jetstream or other wind patterns, so it has a short and low volume monsoon rainy season. Once upon a time there was a green Sahara though, with better winds and good rains.