Not entirely true. There's all sorts of mechanical storage systems, the most common being artificial hydroelectric and flywheels - spin something really fast an

Not entirely true. There's all sorts of mechanical storage systems, the most common being artificial hydroelectric and flywheels - spin something really fast and than extract energy from its rotational velocity later. But a practical issue here is that there hasn't really been much motivation to develop this industry, because there's been relatively little critical necessity for high capacity at-scale energy storage.