I'm a helicopter pilot and yes, I've done 2 deployments. My intent is definitely not to insult or to say we don't have the mental capacity. The Marines I work with do amazing things every day. Every Marine is qualified on the rifle. All O's and many SNCO's are also pistol qualified. But to requalify someone on the range, you have to take them out of their shop for a week. Which doesn't seem like a lot of time, but it is a burden on the daily tempo of getting aircraft flying, and training, which is our primary role for the nation.
Also, again, not trying to insult here, but being qualified on a pistol, rifle, or whatever does not make somebody proficient with that weapon system. Especially if, for most non-combat arms MOSs, means you fire maybe a 100rds that fiscal year. I'm a pistol expert, but I only have requalifed twice in over 5 years. I would love to shoot every year, but time, training, money, and basic job requirements don't allow me to do that.
I definitely do not have the right answer, and your mileage may vary, and so may your branch. I'm just speaking from my small piece of the puzzle. Would I want a weapon in an active shooter incident, your damn straight. But in my mind it's not as easy as just saying everyone take a weapon.