The reason ac is used is that the power transmission is 3-phase. That means that , for a balanced system, i.e. each phase equally loaded, the current in the neutral wire is zero. so you have tree wires, each carrying one third of the power, and you don't need a return wire
You need the forth (return) wire for the case where the three phases not balanced, which is most of the time. Extremely rare to have all phases balanced.
DC systems carrying the same current would require six wires
That's not how it works, AC vis a vis DC. In a DC system the return is via the airframe.