One factor is that games are not realistic concerning violence. I can't think of a single game where the kill would be realistic in terms of how human brain and internals look like or how human in big pain sounds like. Soldiers came from world wars talking about "horror", but modern games are carefully engineered to not give you feels about those killings.

The other factor is that basic training involves sleep deprivation, loss of individuality, some ideological brainwashing, socialization in close group of people isolated from rest of society and other such factors. You can't compare the two.