O F is the freezing temperature of a saturated brine solution, while 100 F was the body temperature of a human. Yes, body temperature has been revised a bit, but the two points were chosen as stable points that anyone could access that would generally be unchanged by pressure changes, etc. Human homeostasis is quite good at keeping a temperature in a narrow range. Also, boiling is massively affected by air pressure. At 5000' elevation, boiling is approximately 202 F and continues to get lower as altitude increases. Lots of people live at higher altitudes. (Hi! I am one of them !)

Edit: I was a little off on the temperature selected for body temp, but still pretty close: https://en.m.wikipedia.org/wiki/Fahrenheit