But you're ignoring the inefficiency of building separate hydroponic operations on individual rooftops and driving around to all of them, etc. Also as SamBam mention above, you don't have to make a simulated sun. Instead it can work as sun -> electricity -> narrow-spectrum-light-that-plants-convert-at-higher-efficiency. You might even genetically modify plants to be more productive with the smallest spectrum possible.
Because it is clearly inefficient to convert sunlight -> electricity -> simulated sunlight.