Sorry for the delay, but I've been underway, and the internet can be spotty out there. I think it is not as uncommon in real life as you think, but I do agree that in fiction it is overdone and done poorly in most instances.
I fired a gun one time before I joined the military, and that was just to shoot a zombie target for fun at a range. When I had to qualify at my first command, I discovered that I just have a natural ability to shoot. I regularly shoot perfect scores with both the pistol and the rifle, including practical weapons courses that involve running around with gear on and changing weapons under a time limit. Out of 327 people at this command, maybe 5 of us can shoot perfect scores, and the other 4 shoot a LOT outside of work. I have a weird stance, and I look like an old man when I shoot, but it works for some reason.
As to the other point, I have seen people who have never been in life-threatening situations before (even in the military) react like it's just another day at the office, and I've seen people with the same amount of training break down and cry or run and hide in the face of real danger. It is very much something that is inside you, and you can't know which way you will go--away from the bullets or the fire or the missiles, or towards them--until it actually happens.
Now, with absolutely zero training, will the kind of person who will run towards danger be able to defeat trained personnel in a fire fight or a hand-to-hand struggle? Probably not, most of the time. It does happen though. My point is just that training isn't everything. If your default "flight-or-fight" mode is "flight" then you're gonna run every time. If it's "fight," then you're gonna do whatever you have to, and sometimes that's enough.