I've no idea what percentage of serving military personnel opt for a G-Shock, but even assuming it's a high number, going to the gym, washing the car or a mountain bike ride aren't combat.
I think there's an element of babying mechanical watches (just read some of the hysterical "My XYZ has a swirly!", "Is it safe to wipe my ass in my XYZ?" posts if you doubt it!) that leads people to believe they
need a super tough 'watch' for all those roughtie-toughtie combat-like experiences their life throws at them.
Most modern (at least) mechanical watches are far more resistant to the odd shock than many people giving them credit for and I just take my watches off if I think I'm going to do anything likely to scratch them, but that doesn't include going to the gym or riding a bicycle.
Anyway, it's all a matter of personal choice, I just find the presumption that 'everyone needs a G-Shock', that is often expressed, baffling.
I've quite happily got through a lot of years without one and never had a watch fail as a result (I did put quite a deep scratch in the crystal on a Seiko chronograph I had for many years working on my car once...).
You don't see people stating that 'everyone needs a chronograph' to time the burn on their lunar lander engines, do you?
M