mixing dry yeast, water, salt and flour
The answer is, as you suspected, you killed your yeast. 

If you look at your original description of the process, you add the yeast to flour, along with salt and then add hot water, the flour is presumably room temperature, so it acts as a cooling agent and spreads the heat out so that most of the yeast survives and the final mixture is about body temp (37 C, or 98.6 F), which happens to be about the best temperature for yeast to grow. You do however need a little warmer to activate dried yeast, but this will be only a matter of 40 C/105 F

When you do this with just the yeast and water: You say that you use water that is as hot as your finger can stand. This happens to be about 55-60 C/130-140 F, yeast are killed by water hotter than about 45 C/120 F. To determine the tmperature, do not use your finger, these tend to be less heat sensitive than other body parts, instead drop a little on your wrist, if it feels comfortable, then it is good to go. If it feels hot the water may be too hot.

You can indeed just mix the flour, yeast, salt and add warm water for an instant dough, there is often no need to proof the yeast (this is the water step). The proofing step does lower the risk of your dough not rising because the yeast were inactive though, and then you have wasted ingredients...