I think they wear it to protect their skin from the sun first and foremost, and the choice of fabric is then designed to alllow for good air circulation and evaporation. At least, that’s the point during the day, but it’s worth remembering that it can and does get damned cold at night, and the same garment works well then too. From a purely physical standpoint, If the sweat is evaporating from your skin, it is that action which moves heat, not the length of time the liquid stays on your skin. It’s a matter of the enthalpy of evaporation, so a high rate of evaporation can only be a good thing. A good light fabric to protect you from sunstroke and dust, breathable to allow for (ideally moderate) perspiration to evaporate rather than drench the fabric, and then the whole thing works to keep you warm at night. If you’re constantly on the move, it’s pretty efficient!