[PeterisP]

A key difference from your examples is that the enemy doesn't have the capability to simply (and risk-free) send an arbitrary number of bullets at your soldiers' hearts or RPG rounds at your vehicles; the major "cost of delivery" is not the bullet/RPG but the trooper (and his risk) bringing the gun to get a chance to make a targeted shot with the bullet/RPG.

If bullets could leave the factory and magically fly straight at your soldiers, expensive single-use ballistic plates would not be a practical solution - you'd simply run out of them as you can't possibly produce as many of them as the enemy can make bullets.