I'm not talking about being trained in how to use the gun, how to aim accurately, etc. I'm talking about how to think, act, move in a crisis, when seconds count, and not panic. This is not the kind of thing that civilians are normally trained to do, unless they are in certain fields (EMS, LEOs). It's something that the military tries to train you for, and combat operations throw you right in the thick of.
To put another way, even with training, a lot of soldiers tend to freeze the first time they found themselves in combat, because that's the natural human reaction. This is why they're trained to act, but also to follow orders, so the more experienced NCO can get them moving, firing back, until they overcome the initial reflex response.
It's certainly not true of everyone, but I certainly wouldn't want to rely on Joe Smith who's taken a few gun safety classes and goes to the range every other week to react in the same way that a military combat veteran would/could.