As a nearby comment mentions, building regulations had been designed to convey rain water to the sea as fast as possible, due to a justified fear of floods. At the micro level, this means diverting rain water to concrete channels (e.g., streets or drainage channels). For instance, when I put in a building in my back yard, I had to install a pump to pump the water from a tank filled by the rooftop gutter into the street, rather than letting it soak in to my yard.
Now, multiply that by millions, and you can see we're wasting a lot of water.
Changing the whole conception of storm water ("collect and dispose of" -> "disperse and soak in") involves changing building codes and city/county/state infrastructure practices.
Because ground water is all externalities, there is no ready market solution.
Because there isn't as much money in it as in ginormous megaprojects? E.g. the "train to nowhere"[1] in California had the support of a Teamster leader because:
The legislators in Sacramento got it right
when they voted to fund this project since
it will help hundreds of people find good
union jobs
and the high speed rail line will provide
a viable alternative to car and plane travel
in the state.
Follow the money.[2]:
a catchphrase ... which suggests a money trail
or corruption scheme within high (often
political) office.
[1] http://www.laweekly.com/news/california-bullet-train-to-nowh... [2] https://en.wikipedia.org/wiki/Follow_the_money