1) The safety mechanisms on say a glock are different than on a lot of military rifles soldiers are trained on. An Ar-15/M16 can go off without pulling the trigger if the firing pin gets stuck in the channel. That won't happen with a glock because the safety physically blocks the primer from being struck. Also in theory a free-floating firing pin could maybe somehow get slammed hard enough or slam an abused primer enough to set off some military rifles.

2) Some hunting shotguns or military rifles aren't drop safe. Modern handguns are.

3) A military rifle or hunting rifle generally has the trigger exposed at all times you are carrying it. A CCW handgun, you are not exposing the gun and trigger unless you are about to shoot someone.

Now I've never served in the military, other than a rag-tag Kurdish militia. What I would imagine the boot sergeant or whatever they are called do, is tells the soldier they will keep the manual safety on or the weapon unchambered and leave it at that, because explaining the intricacies of a striker-fired pistol vs an M16 to a bunch of barely out of highschoolers from Guam who are already exhausted from sleep deprivation and jarring work-outs would not be terribly productive.