Why is the body of the car used as a return path for current flow?

This wiring method is used in cars and other vehicles because it saves the need to use expensive copper wires for the ground return side of several circuits in the car, such as tail lights, stop lights, rear windscreen wiper, etc.

ASE certified mechanic's answer

Most vehicles use the ground or body of the car for return paths. It is not considered a bad practice as it is done almost exclusively for vehicles as it saves wiring. More and more vehicles are using computer controls that utilize grounds to operate accessories. The most common application is a door switch that turns on a dome or courtesy light. When the switch closes (door open), it grounds the wire to the body and because the computer senses the wire grounded, it turns on the courtesy or dome light via a relay.

If you look at the negative post of a vehicle you can follow it to the body, chassis and engine grounds to prove they still utilize the body and chassis of the car for the ground return.