There is a pretty good case to wean us off cereals. They aren’t the best diet health wise. We only consume so much because they are calorie dense and easy to grow.
So... most of humanity on a more-or-less vegan diet.
And 'easy to grow' is rather dismissive of the vast amounts of progress we've had to make to come to the point we are now...
So you take more land area for solar, why wouldnt you just plant crops on that land area instead...
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.
"The investors funding these enterprises do not understand agricultural economics. I keep investigating these stories looking for some kind of 'breakthrough tech' that would dramatically reduce costs or increase yields. I've been tracking these vertical greenhouses for fifteen years, they open and a couple of years later they're gone."
Higher productivity per watt in the broader industrial sense does not equal higher productivity per watt in the smaller sector that is agriculture.