Because people are really, really expensive to employ. The fewer people it takes to generate 1 kW of energy, the cheaper that energy can be sold on the market, and to a first-order approximation, economic growth and increase in standard of living are mostly because energy got cheaper (and these things stop happening when energy stops getting cheaper). Note that for most other goods, it's uncontroversial that the reason people can afford it now, thus bettering their standard of living, is that it takes fewer people to make them than it used to (because of automation).