Because the rotation is caused by a force at the rotor tips, so there is no torque on the aircraft. A conventional helicopter has an engine that is rotating a s

Because the rotation is caused by a force at the rotor tips, so there is no torque on the aircraft. A conventional helicopter has an engine that is rotating a shaft connected to the rotor, so the engine is creating a torque on the shaft which causes the rotor to spin and then the shaft is creating an equal and opposite torque on the engine that is causing the aircraft to rotate the opposite way, which the tail rotor cancels.