It might sound like bad fuel efficiency but the engine efficiencies they have managed to achieve are insane. The latest Mercedes and Ferrari engines are able to gain 47% efficiency [1]. Compare that to the road cars which are able to do 25-30% thermal efficiency and Prius which can manage 40%.

In my opinion, the fuel efficiency suffers because it's a sport at the end of the day and burning more fuel at a higher rate (limited to 100 kg/hour) gives you more power.

[1] https://arstechnica.com/cars/2016/05/turbulent-times-for-for...