Actually, while you are in the car, you have no charge (assuming you are touching the key in the ignition or some other metal part.
The car picks up charge as it moves through the air, and probably some interaction with the tires.
When you step out, you discharge the car. It isn't actually you that has the charge built-up. So what you need to help keep the car discharged is a light wire or chain hanging down.
There are two theories, and you represent one of them here:
"The charge buildup is between the car, and the rest of the world. You get zapped, because you form the path to discharge the car"
The other one is:"The charge build up is between you and the car. When you get out, you separate the charges, increasing the voltage. The discharge is between you and the car"
I believe in the second explanation.
(And I have had ground straps on cars. In my case, they didn't work at all)