I would say that Rob's discussion above is probably the reason why, but the benefits of the training would be useful, namely carrying larger loads with less fatigue, increased postural stability should allow for more accuracy (anyone who has seen the surprising results of Akuzawa shooting with minimal experience can attest to that).
I understand what you're saying and I agree that all of those attributes would be beneficial to anyone who has the time to put into IT. Heck, I'd like to learn just so mowing my lawn wouldn't be such a pain!
But in reference to the military; from what I gather it takes up alot of time to even establish a solid baseline foundation, much less get really good at this IT. Time which is spent on doing more important things like functioning under pressure, etc.
And on top of all that, how many people re-enlist after their first enlistment which averages about four years? Not too many in my experience.