Except that top-flight militaries are ready for them. All of our armored vehicles are already air-tight, and hazmat suits are widely available throughout the military. It doesn't help the chemical weapon's case that the countermeasures to it fall into the broad category of Nuclear/Biological/Chemical, all of which need roughly the same countermeasures (airtight vehicles, gas masks, hazmat suits). Militaries are still equipped and ready for these kinds of attacks.