It wasn't the case that every sheepman like Zhou Tian possessed millions of HP; Zhou Tian could easily dismiss the attacks from Burrow Spiders. But an average sheepman facing the offensive strikes of those Burrow Spiders would, without a proper plan, potentially die within just a few minutes.
Of course, Zhou Tian would never just sit and watch his sheepman soldiers perish on the battlefield. But under the current circumstances, he couldn't come up with a viable strategy. After all, the Burrow Spiders' attacks were composed of poison, hardly something that could be ended quickly, and the afflicted sheepmen didn't have the stamina to hang on for long. Despite possessing the Life Spring Water that could resolve this crisis, Zhou Tian simply lacked the time to take any sort of action.