They don't, it just seems that way because everything that can be used for war is used. There's a mad rush to productize existing inventions.
People tend to focus on wars during discussions about history so their time periods are overemphasized.
How is it then, that inventions seem to accelerate during times of war? WWII alone saw the invention of, or massive improvements to rocketry, the jet engine, cruise missiles, Radar, and the Atomic bomb