Large airships wouldn't usually fly that low, at the beginning of the war almost no plane could even reach their cruising altitude and AAA wasn't really a thing yet. Even later on in the war when planes could reach them it took a lot more than a few bullets to take one down because they're not like balloons with only one segment e, but instead they're a bunch of armored and enclosed segments linked together, like a ship with a watertight hull and separate flood compartments.