AC transmission just requires live wire, why is neutral present
Because you can die with 15 to 45mA through your body across your heart. The earth can conduct current at that magnitude to kill you!

However if you try to power a real world load at 1amp, 10amp or even 15amp the earth resistance will create so much voltage drop at those currents that there would be little voltage across the load that it would fail to operate.

The neutral offers a very low resistance path back to the power source such that your load sees most of the source voltage across it that it will function properly.