I always lock my car doors. My mother was once coming home late after work in the Winter so it got dark early and got stopped at a train. She always locks her doors too, but some guy did try to get in her car and he tried all 4 of them, I shutter to think what could have happened if she didn’t have the doors locked, as she was blocked in with nowhere to go. I carry a gun in my car and I might have it at the ready if I had a bad feeling or saw someone shady approaching. Even if some carjacker had a gun, I might drive off anyway and take my chances, there’s no guarantee he won’t shoot me even if I open the door to let him have the car.
I don’t lock my car doors while driving…hell, I don’t even lock the HOUSE doors, even when I’m going out somewhere. 
Mind you, carjackings are a rarity in Australia…so rare that they actually make the news when they occur. And mostly it’s when a woman with a kid is putting the shopping in the back of her hatch or SUV. Random carjackings involving people driving are REALLY random.
The same way people are robbed on the street. Someone approaches you in a blindspot with a gun and wants the car, give it to them. That’s what we pay insurance for. Who cares, give them the car. A material thing which can easily be replaced with a check from the insurance company. I’ve known people who fear parking their car on the street because they expect it to be stolen while they are having dinner. I’m not going to go through life worrying about someone taking my car.
It’s just very touchy. No one is going to speed off in it…they may lurch off.
Well I know if you were to speed off every time someone approaches your vehicle you are likely to commit a completely different crime (ranging from minor traffic violations to felony negligence or endangerment) - your presumption places that car over someone else life and possibly your own. Another thought, besides your question being completely ridiculous is that you cannot control other people or their actions (you saying you don’t LET someone walk near your car would be to say that you are able to know every approaching person within a good radial distance from your car and that you can control the unknown). There are several scenarios and methods making it possible for another person to steal your car whether you were in it or not and regardless your mature capability of locking the doors.