I'm not a physicist or electrical engineer, bit I'm pretty sure it's you, or rather the clothes you wear, and the seat material.
You inevitably slide across the seat when you get out. If you have certain man-made materials in the thing covering your bum, you rub the two surfaces together, generating a static charge. Like hair and a balloon.
This charge has nowhere to go until you earth, there is a potential difference between you and the ground, which you earth as soon as you touch a conductive surface. This is usually your car door.
You'll notice you get the shock getting out, never getting in. Blame the plastic seat material, and whatever you choose to cover your rump. To a certain extent your shoes too I think, if they have really thick soles, again the charge has nowhere to go!