Why have the neutral wire in house current
As a direct answer to your question:
Just connecting a conductor in a different potential to ground does not "complete the circuit fine", unfortunately. 
In multiple locations, the current carrying capacity of the soil is very bad, as there are no guaranteed specifications for the electrical characteristics. It can be 100% bedrock, for example. Bedrock not being a good conductor (1mS/meter = 1kOhm/meter)

Other things: 
Over here we have a 3-phase system, not 2 (implied in your 180degree secondary phase)
I'm not quite sure it would save them that much money, even if it worked. I believe most of the cost would be in clearing the path, putting up poles.

-Source: EE student, designed mock-up wiring for houses. Not a grid designer, though.
If you have anything to correct me on, feel free to!