From someone who lived in a place with very dry dessert like climate and moved to one that is next to the sea. It is all about humidity. In most dry climates your biggest problem is direct sun-contact. Sun will literally start to burn your skin as soon as you are out but you will be fine in a shadow. In a humid climate sun will not burn as much. But air becomes very heavy and you start to sweat non stop. So in dry climate you want a thin, preferably white clothing to prevent sun contact. While in humid climate you want as little clothing as possible.

Hovewer if you are gonna be out in sun for a long time especially in summer. You should cover your body at least your head and your body if you don’t have a sun screen. People living in forest doesn’t need to do this because they will mostly be in shadows from all the trees.

I don’t actually know but maybe it has to do with high humidity favoring less clothing so your sweat helps cool you more effectively? Or perhaps the higher ease in finding shade in the tropics.

I’m not an expert so take this all with a grain of salt but tropics have more trees and shade. Deserts have none.

Humidity is a thing too. Deserts are dry. You sweat less. So the clothing works and keeps you cool. In the tropics id imagine it sticks to you.