When the aircraft is walking on the ground, it is realized by the reverse thrust of the jet engine or the propeller on the air. The thrust is maintained at a stable value and does not accelerate. This is the principle that we see the aircraft walking on the ground.
There is no separate drive system for the wheels of the aircraft to travel. The fuel consumed by the jet engine or propeller is much larger than that of the tractor. It is more economical to use the tractor than the aircraft itself.
In addition, if the aircraft consumes fuel when it does not take off, it will shorten its emptying time. If the tractor is used, it will not consume fuel and extend the flight time and flight distance of the aircraft.
Other aircraft, due to their own structural reasons, such as helicopters, Antonov An-225, Airbus A380, C-130 "Hercules", etc., can not use the engine to walk on the ground, must be towed by the tractor.