Neither does anyone else, its one of those archaic units that changes slightly based on who is using it and hangs on in oil and gas industries, and also air conditioners and heaters.

It was defined as the amount of energy to raise one pound of water by one degree Fahrenheit at one atmosphere of pressure, but that amount of energy depends on the starting temperature of the water, and different things use different starting points, so it ranges from about 1054 to 1059 joules

https://en.wikipedia.org/wiki/British_thermal_unit