The sun produces a full spectrum of light, like a blackbody of a certain temperature. The atmosphere then selectively absorbs, scatters, or reflects a portion of it. That produces the Earth daylight spectrum, which varies by season, latitude, and time of day. It doesn't ever match up precisely with the absorption spectrum of plant pigments.
A grow light can produce the exact wavelength of blue light most readily absorbed by chlorophyll, and the exact wavelength of red light most readily absorbed by phytochrome, and the exact wavelengths of other hormone/signaling photoreceptor pigments.
The grow light produces exactly the light that the plant needs, whereas the sun throws out a lot of energy that the plant cannot use effectively. If we had a material that could absorb two green photons and emit one blue photon, that would improve the photosynthetic efficiency of sunlight, but for now, it's still easier for us to turn green light into electricity and power a blue LED.
A grow light can produce the exact wavelength of blue light most readily absorbed by chlorophyll, and the exact wavelength of red light most readily absorbed by phytochrome, and the exact wavelengths of other hormone/signaling photoreceptor pigments.
The grow light produces exactly the light that the plant needs, whereas the sun throws out a lot of energy that the plant cannot use effectively. If we had a material that could absorb two green photons and emit one blue photon, that would improve the photosynthetic efficiency of sunlight, but for now, it's still easier for us to turn green light into electricity and power a blue LED.