I suspect capturing runoff is not nearly as simple as it seems. In SoCal/LA it rarely rains, so when it does rain a few times a year the runoff is a black, horrendously toxic stew of petrochemicals and urban detritus. For a day or two following a rainstorm there are advisories to avoid swimming at the beach because all that crap goes into the ocean and makes it hazardous.
I imagine it would be incredibly difficult/expensive to treat that water even to graywater level, let alone potability.
Certainly by the time the water enters public catchment basins or the LA River it's nasty. TFA envisions harvesting and storing water onsite. I'd imagine the most common method would be attaching gutter downspouts to a really big tank, although better landscaping and less hard ground cover could also help.
those advisories, at least in SF and a few other placesx are often the result of too much rainwater in the sewage system and so raw sewage and runoff both end up going into the ocean.
I imagine it would be incredibly difficult/expensive to treat that water even to graywater level, let alone potability.