The difference is that humans emit their own heat. Combined with our funny tendency to wear insulative clothing that can asymptotically approach zero net heat exchange with the atmosphere, acceptable temperatures skew wildly towards and beyond freezing.

Meanwhile, without some kind of acting cooling mechanism, any temp even slightly above fever temp is inevitably fatal. You can only take off so many layers. What are you going to do, take off your skin? Sweating helps us humans a lot, but evaporative cooling can only do so much to reverse the heat gradient.

50 F is excellent... with a light jacket or a blanket. Not so much if you're naked.