Much of that is dependent on the wet bulb temp, which is where some people are crying about high humidity at lower temps. But in most deserts the humidity also tends to be quite low, so the wet bulb temp can be fairly low at air temps quite a bit higher than 100F because sweat evaporates quickly.

But yah, motorcycles at speed are another thing entirely because sweat/etc can just as easily blow off as evaporate. And so, you can't drink/sweat enough to be cooled. Combined with the need to wear protective gear which also tends to stop evaporation is a deadly combination. I ride mountain bikes in 100F+ weather and its another set of dangers, and one of the best feelings is dumping cold water through the vents in a helmet and feeling the burning hot water flush down your face and be replaced by the cool. But again, i'm basically carrying a significant quantity of <40F water intended to be sprayed on my already sweaty self, along with a 32F water i'm drinking not for hydration but as coolant.