> Plants, on a large scale and reasonable times, generally can't construct themselves

Compared to manufacturing PV panels they do. Like the scale isn't even comparable. A handful of people can farm a couple thousand acres, where an equivalent solar farm would be on the scale of the largest solar farms in the world. The power output wouldn't be similar, but in terms of effort involved in covering a large area with power generation, the speed you could grow plants is like orders of magnitude in difference. Not to mention that individual plants can provide thousands of seeds which you could then use to propogate thousands more acres.