At least partially because you don't have to reproduce the whole spectrum of sunlight, but just those portions that are used by the plants for photosynthesis. E

At least partially because you don't have to reproduce the whole spectrum of sunlight, but just those portions that are used by the plants for photosynthesis. Even after solar cell efficiency and other losses, you might still be able to deliver more of the light you want to the plants, in addition to being able to control the day/night cycle.