Because freedom.

In all seriousness though, temperature is the one unit where I find imperial far more useful than metric.

In everyday life, outside of cooking or scientific endeavors, 0-100F is far more useful than 0-100C. 0F is really cold, and 100F is pretty darned hot. And we spend most of our existence living between those numbers. Versus -18C to 38C? Having a simple 0-100 scale for that seems to make the most sense.

Sure, it gets both colder and warmer, but in terms of actual usage, I'd bet 90% of our general temperature usage is to relate the weather. You just get far many more datapoints with Fahrenheit with which to reference and compare.