Why is jet fuel measured in pounds instead of gallons?

Fill up your car at the gas station and you're buying it by the gallon. Refuel your Boeing 747, and you're measuring fuel ...
The New York Times

European Airlines Face Fuel Shortages Within Weeks

If tankers do not soon begin crossing the Strait of Hormuz, airlines in Europe may not have enough jet fuel to operate all of their flights. By Niraj Chokshi The war in Iran has forced airlines to ...