Why don't we get shocked when we touch a car?
You don't get shocked because you're talking about DC with no connection path back to the positive terminal of the battery.

If you touch something metal on the vehicle you're now at the potential of the negative terminal of the battery (or the car's ground voltage). If you're also touching earth ground, then a very slight current will flow to balance out earth ground and car ground. But after that's balanced, there's no difference in potential across you and no current flows.

It's kind of akin to a bird resting on a high voltage power line. They don't get shocked much because there's no completed circuit.