I sure have noticed commercial aircraft making contrails, but I know what you mean about military aircraft in general. Here's my guess: military aircraft fly at weird elevations, at elevations where condensation of water into liquid or solid form can readily happen. They do this for a variety of reasons, but the big ones are that they need to fly, and train to be able to fly, at elevations where incoming military fire is less likely to find them, and that means flying low. Commercial aircraft spend most of their airtime in the stratosphere, where there isn't enough moisture to form contrails.