that's the chart i was looking at. you're cherry-picking years as well to get that answer. looking at all the years instead, the usa starts out at 94, peaks at 104, dips down to 87, and then rises back up to 97, with lots of wiggles in between. that isn't a picture of a secular decline in energy use, it's a picture of random fluctuation around an average. the others are largely similar. the main outlier among 'developed' countries is not south korea but the people's republic of china, though slovenia deserves a mention for falling 40%
why? what holds the usa line so steady? probably partly the usa has lost the ability to build not only high-speed trains, subway tunnels, highways, and leading semiconductor fabs, but also electrical infrastructure, but two thirds of energy use is non-electric, and other countries are succeeding in building trains and whatnot
the price of energy has not been holding steady during this time but rather going up, although in the last decade that is starting to reverse. perhaps the reason consumption-oriented energy use has gone slightly up rather than way down is the efficiency improvements you mention
[the following paragraph is completely wrong, and i appreciate the correction from h0l0cube. therefore so is my claim that the measure itself is cherry-picked. i had just misunderstood it]
this measure, incidentally, doesn't count energy used for exports at all. so if the us and japan are producing an energy-intensive good independently at the beginning of the chart, and then in the middle the us starts importing it from japan, the usa's line will go down while japan's remains steady, because japan's added energy use is for export rather than for consumption. that's what i mean about it being a cherry-picked measure even for those cherry-picked countries