Why don't grocery stores grow vegetables on their roof?

Seriously. Why don't grocery stores do this? Obviously, they have a lot of underutilized sq. footage available. They could set up automation/sprinklers to handle a lot of daily/mundane tasks (hydroponic) . It seems like it would take minimal labor -- I would think the labor would cost less than the fuel required to ship produce. Cooling costs (for inside) would likely be lowered. You could get fresher, higher quality and cheaper vegetables.

Home depot maintains their garden area with specialized gardeners who are responsible for watering, fertilizing, etc. Tomatoes, squash, cucumbers...these are all really easy to grow. This seems like a slam dunk. Why don't more do it? Is there a weight/support issue with these roofs? It seems like hydroponic systems wouldn't weigh that much.

LINK

Here is what these guys do (they PAY rent for the roof space and the architecture/building required to prepare and support the roof!):

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"We come in on the rooftop as a tenant of the building. We rent the rooftop space. We pay for the upgrade, the insurance costs, the fixed costs for planning and development and the soft costs of architects, etc. We take all of that on. We outsource the equipment. We don't invent technologies. We're taking existing proven technologies and applying them to this rooftop. Then we make our money off the sale of the produce. The technology is controlled-environment greenhouses, year-around systems keeping constant temperatures and controlling the environment there. No pesticides, no herbicides, all integrative pest management systems and composting and trying to use paper and food waste from the building as the nutrient stream for our plants."

A typical project covers about 20,000 square feet -- about half an acre -- and fairly efficient, says Agoada. "Our growing techniques use somewhere between 5% and 10% of the water that they're using to grow lettuce out in Salinas Valley," in California's Central Valley, considered the nation's breadbasket. Given that around 80% of water use in the state goes toward agriculture -- and about a fifth of the state's total energy use goes to move and treat water -- such efforts could create significant water-efficiency and greenhouse gas benefits, should the Sky Vegetables model catch on.

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