> "humans shouldn't live here" hot.

A bunch of it has to do with humidity levels that impair our natural ability to sweat. If the wet-bulb temperature [0] is high enough, a place may be unlivable without artificial cooling.

[0] https://en.wikipedia.org/wiki/Wet-bulb_temperature

Florida gets hot, but not wet-bulb-above-95F hot. The proximity to the ocean keeps it from getting quite that high. People lived there before AC.