The problem is when water evaporates and come as a rain, it is distributed. And, we need large amount of fresh water in one area. Additionally, you are not cons

The problem is when water evaporates and come as a rain, it is distributed. And, we need large amount of fresh water in one area. Additionally, you are not considering the time closed loop system takes to replenish the water source. I agree with your last point. However, I think they still can filter and make it harmless, but we know there is no incentive to do that and the cost is high.