Though it is so hot in the desert, why do desert people cover themselves from head to toe?

Here is some advice and a short explanation - In the desert, wear white clothing. White reflects heat. Even the Turags, a desert people famous for their blue-black clothing, wear white outer garments occasionally. Next, cover your entire body. The reason desert people completely clothe themselves, even in the heat of the day, is because it slows the rate that sweat leaves the body. In other words, ration sweat, not how much you drink.

http://www.desertusa.com/desert-activity/desert-survival-tips.html

To protect themselves from direct sun exposure and sudden sand storms while allowing body heat to escape through the natural fabric. The layers of loose clothing also slow down the evaporation of sweat, keeping the body cooler.

Clothing selection for desert travel is somewhat different than for most other wilderness activities. The less exposed skin, the better. While cotton is not good for most cold, wet climates, it is useful in the desert. Light colored clothing reflects sunlight and lessens skin temperature. Ripstop cotton (cotton material with nylon threads latticed within it) is best because it resists rips that are common in the desert. It is light enough to allow heat to escape, does not create a clammy feeling in low humidity, and protects against some ultraviolet radiation exposure and blowing sand. If ripstop cotton is not available, then any tight weave cotton is adequate. Long sleeves and long pants are a must to protect against spines, thorns, splinters and insects. More importantly, they protect against solar radiation that causes sunburn and increases body temperature, and trap more cool air next to the skin. Sweat that is trapped may decrease water loss via evaporation.

https://aneskey.com/desert-travel-and-survival/

Those long silky, thin robes help block out the sun a little and mainly the air flows underneath and cools the body.

The tunic is loose fitting, allowing air to circulate, preventing sweat from evaporating too quickly and slowing dehydration in hot, dry air.

http://www.pbs.org/lawrenceofarabia/revolt/clothing.html