Because the ground is there already, and is already connected to all houses and all power plants. If there was a really massive ground fault and current started flowing to ground somewhere, the electricity could indeed flow back to the power plant that way. In fact telegraph engineers back in the day initially set up their circuits with a return path, but then realized that the system worked just fine if they used the ground as a return path, and halved their installation costs. Of course, with modern systems, if there is a massive ground fault there’s hopefully equipment for detecting and interrupting that (breakers, for example) so nobody gets electrocuted.
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