Put another way, I can sit in a 190F sauna for a half hour, but a half hour out in the sun in, say, Singapore, feels awful to me. 85F where I live now (unusual but happens), in a relatively dry climate, is bearable, while the same temperature where I grew up, in a much more humid place (where it was more common), feels gross and saps my energy.
And I can be out snowboarding in 25F weather that's calm and sunny, and feel fine in a medium-weight clothing. But add clouds and wind, at the same temperature, and it's unbearable.
And to me, based on the temperatures I usually see, 15F is waaay too cold, and 90F is waaay too hot. Sure, I guess I could round that down to 100F and up to 100F, but that just feels like being arbitrary for the sake of easy numbers.
Food, saunas, the weather, a fever, and much more are often over 100F.