What about when you have yous two kids strapped down in the backseat?
Even if you are carrying, being in a crowded parking lot and indeed inside your own car present many problems if you want to follow rules like "know your target and what's beyond it."
At the risk of getting flamed, it is likely (especially if you have passengers) you may not want to open fire in the middle of a fist fight while you've got a steering wheel with which to contend and possibly a seat belt limiting your mobility. Indeed, many people train with their firearm in simulated bad guy situations - how many of us have trained for a car jacking?
I'm not saying don't carry. Indeed, the firearm may be helpful once your outside the car, That being said, there are MANY ways to train and be "armed" for a carjacking. Implements aside from a firearm for disarming bad guys reaching into your cockpit abound. This is especially important if you have children in car seats or elderly parents who ride shotgun (no pun intended).