For anyone who has ever considered the way modern militaries are trained, the answer is sort of obvious for why this is. The military training doesn't enhance your thought process, it breaks it. You are being broken down, and rebuilt. The parts of your brain that make you hesitate before going against orders have been actively targeted for eradication or suppression. (You are pushed and pushed and pushed, and if you fight against the push, more weight is added to what you have to do) If successful, you are no longer an individualist, you are a collectivist. (They stop punishing you as an individual and start punishing you as a group. You don't just hurt yourself when you disobey, you get all of your friends hurt too.) You have a new family, and that is the people who have gone through the hell you've experienced and come out the other side. (The tendency to bond with people who have gone through the same shit you have is used here to cement a unit together)
Resentment is then built between you and your supposed 'brothers'. Now let's step into the brain of that soldier:
Fucking civilians thinking they know what's good for the world. They don't understand what it's like to go through the hell you've experienced becoming a soldier. They think they know what it's like in the 'real world' but they've never really seen what hardship is like, have they? No, the only people who have actually been through what you have are your brothers in arms. If they want to make it into a fight, we can make it into a fight. That's what I was trained to do, right?
Obviously this can't apply to every soldier. However, this is the
ideal position of a soldier, and what people who are paid to train soldiers are trying to create. The bottom line is that they want to get the most bodies from the state of "I'm afraid of killing someone" to "I will not hesitate to kill someone if I'm ordered to. Or if you make me." That's the job, and those are the qualifications, and damn if they aren't good at getting those results.
There's a very good reason why people who have been trained to kill have a difficult time transitioning back into civilian life.