Kilowatt-hours are so unnecessary

WHY do we measure energy in kilowatt-hours? A watt is a joule per second. Energy divided by time. You multiply that by time, and you're back to energy again. Why don't we just use the SI energy unit, the joule? One kilowatt-hour is 1000 x 60 x 60 = 3.6 megajoules. I could understand if it were simply a different system, like feet vs. meters, but watts are derived from joules so there is no reason not to convert back.