Easy to see why with some quick physics.
edit: those copy paste formulas came out with terrible formatting. basically 2 main forces, rolling resistance and drag. rolling resistance is linear but drag from wind is cubed with speed.
30 km/h (8.3 m/s): ≈ 0.40 kW
60 km/h (16.7 m/s): ≈ 1.74 kW
80...