Expanding on this: because the neutral current is close to zero, the conductor for the neutral current has looser requirements on its impedance. With perfectly

Expanding on this: because the neutral current is close to zero, the conductor for the neutral current has looser requirements on its impedance. With perfectly balanced phases the neutral conductor can be eliminated, saving money and material. In practise, the Earth itself gets used as the neutral conductor. This is why one sees three wires in transmission lines and not four. Incidentally, the closely spaced multiple wires within each conductor of an electrical transmission line, typically held apart by spacers, act to reduce the inductance of the transmission line, which reduces losses. The little "dumbbells" which are on each side of each tower, are vibration dampers to reduce metal fatigue.