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.