9 hours ago, vom said:Then why can't we fill the jar with water?
You can, from toilets, from water you find in kitchen cupboards, from dew collectors. Jars are just not shown as a separate item, like gas cans. What you collect is one quantity of water and you don't need to concern yourself with how that water is transported.
Think about it, it was always illogical that there are jars of water in the toilets, but everyone accepted this as the truth. The best explanation to this must have always been that you have containers with you, jars for example or anything else, and you fill these containers from the toilet.
If you are asking why you can't get water from lakes, that is the solution TFP came up with to make water a scarce resource even though lakes are everywhere. Think of the water as simply unusable and don't think any further. For what TFP wanted to achieve it was the easiest solution. Whether there are better solutions would have to be evaluated.