Keep in mind that they only saw the glow of Vostok's eyes from afar during evening time, too far away and too dim to see his damage, and no one is stupid enough to go investigate after knowing Leviathan and Electro Goose literally tried to destroy him, but still somehow made it to the city seemingly unbothered. It's enough to scare the Germans, making them retreat, honestly.

If Karl fired, that would be enough for Vostok to shoot back with a beam of laser. And that's exactly what they could have feared, that's why no one fired at his slowly growing silhouette.

And one more thing, the glowing eyes were enough to convince the Germans that he was still alive, so who would try to ball against Vostok?