Why does it seem like desert cultures tend to cover most of their body, while tropical cultures tend to wear very little clothing?

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.

In the tropics your problem is heat and rain. Any material in the tropics will be taking in water, and when it rains it rains so hard your eyes will hurt from all the rain rubbing against them. Thin clothes dry fast, and no clothes is twice as good. If the sun is too hot just take a siesta.

In the desert, wind is your enemy because it sprays loose matter everywhere. Hence tents and flowing clothes for protection.

If you're farming in the tropics you want long sleeve thin clothing to limit Sun damage. Broad hats are also ideal.

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'm no scientist but as someone from a tropical country, it has to do with humidity and covering yourself would make yourself sweat more. Also look up wet-bulb temperatures

As someone who lives in the tropics, humidity.

The air is humid here and sweat does not seem to evaporate quickly, the dew point here is also high. Covering yourself up like those in the Desert can be a bad idea when you are sweating, you would want to evaporate all that sweat as the evaporation seems to take all that heat energy out of the body.

There's definitely a functional reason based on physics for this. I find I'm hotter working out without a shirt on, that I am if I have a loose shirt or singlet on.

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.