Its very ressource efficient to clear water in a water treatment plant. Way better than every household having to purify water themselfs by boiling it. And now that we already have it it would be just stupid to build a complete second system just for dirt bowl water.

What people can do and what is becoming quiet popular is to buils tanks for rainwater and use that to do stuff like flushing toilets or watering plants. But that rather little bit of piping (when building new, retro-fitting is a pain in the ass) costs very little. Waaaaaay better, cheaper and efficient than building an entirely new network.