see my comment about overhead costs. However there are a number of interesting details: since with LEDs you can stack plants vertically, the horizontal area is less important. You can also keep things more compact (plants in a field aren't particularly dense) with light coming in at multiple angles leading to bushier plants with more growable surface area.
I guess the real question is, "what are the amortized costs of growing using solar panels and LEDs compared to fields", and that's a super hard problem to solve/estimate. So far, people have only been growing high-profit crops, so it seems like the economics don't favor interior growing yet even for well-heeled players.