Generally the idea of caches is that they contain a small subset of all your data in a type of storage that's more expensive, but faster. The extra complexity of primary databases is largely down to the fact that they have to use disk storage.

BTW, you're way off the mark with the numbers in your car analogy! I know the precise numbers are not the point, but it's really not 90% vs 25% efficiency. The oft-quoted figure of 25% efficiency refers to thermal efficiency. It's a measure not of how much power is wasted by the car, but how much of the energy in the fuel is turned into useful work. To compare electric cars, you have to consider how the electricity is generated. It's mostly generated by burning fossil fuels. It's more efficient than petrol cars because it's done at such a large scale, but it's not 90%. It's more like 40-50% afaik.