It goes back to the transformer via the service neutral wire. It's a closed loop: from the transformer to the house on either hot wire, through the loads in the house, and back to the transformer on the neutral wire. That explanation is highly simplified to the point of almost being wrong, but it's an easy way to understand the general concept.
During normal operation, there is no current flow to ground. The only time current flows through the ground is when there is a serious problem. The path back to the transformer always has much lower resistance (a fraction of an ohm) than the grounding electrode (tens or hundreds of ohms). The ground connection is not useful as a current return path.
