Evolution of the Mom Arm

This morning I was driving my son to practice when I had to slam on my brakes to avoid hitting the nice person in front of me who decided rather suddenly to corner on two wheels and to conserve their turn signal life by not using it as a warning to oblivious drivers behind them. In a split second I hit the brake pedal and threw my arm across the car to keep my child from flying forward. Why do we do that? Is it a developmental reaction that has evolved since the invention of the Model T? I can see how the instant arm of protection could be useful at slow speeds back in the old days when no one wore seat belts that glued them firmly in place in a stop short situation. It wouldn't be useful in a collision though, except maybe to keep one's purse from spilling into the floor board. I've noticed that I thrust my arm across the passenger seat even when I'm in the car alone. I usually manage to keep my handbag in place by doing so, but I'm not sure I am actually thinking about that in the moment of reflex. (this makes me sound like a horrible driver, but I would like to think I'm not. I've only been in two accidents in my adult life) Air bag technology should also have made the flying mom arm obsolete. In fact, if we had crashed through the rear of the maroon devil car in front of me this morning, the deploying air bag would have thrust my arm into my son's nose and likely caused broken bones for both of us. Actually, that might have made me look really heroic. When asked what happened to us, I'd be able to say, "I was protecting my son from flying through the windshield. And it's a good thing I did or he would have much worse than a broken nose!"
That does bring up more questions though about why wouldn't his seat belt have stopped him and why was he in the front seat in the first place. He's quite tall and old enough to sit there, but shouldn't my most precious cargo be in the back, which is really the safest place? Oh! I know the answer. He sits up front so he can hold my purse and keep it from flying off the seat when I slam on the brakes.