Red_Vespa saidBecause we'd be in some place without showers for that length of time. The most you could do was wipe with a wet facecloth, if that.
Okay, but you have hi-tech gear, advanced weaponry... and the one thing the labs couldn't figure out was how to maintain proper hygiene?
I'm sure if more gays and women joined the army, this would be solved in a jiffy. You'd probably even get a tweezer as part of your survival kit
A tweezer won't keep you clean, but nothing says a soldier can't carry one, and some do. Especially to remove splinters and other sharp objects beneath the skin, as I did.
A soldier or marine can be moving rapidly over territory day after day. Base camps may have field showers, but sometimes you're operating ahead of the base camp, which takes time to set up. What are you supposed to do? Identify & seize enemy shower points? Will they accommodate the thousands of soldiers trying to use them in the brief time they are passing through? And in much of the Iraq & Afghanistan operational areas, running water & local showers don't even exist.
An understanding of military operations would help to answer your question. There are "field expedient" methods of removing some of the body dirt, but full showers are often not one of them, notwithstanding modern technology.