It requires building the structure under the greenhouse specifically to support the weight of the greenhouses themselves, plus all the plants, equipment, substrate, and water, plus workers.

Your average existing large-footprint building simply isn’t built to hold that weight - maybe a couple feet of snow - and would need to be heavily remodeled to accommodate it. It also adds a lot to the cost of building new structures, so it’s a sizable investment in either case.

Idk if you’d call that a downside, but that’s why this sort of thing isn’t everywhere.

If people lose the fear of starvation to motivate their contributions to the capitalism machine, then what??? How dare you try to destabilize our massive profits with something as heinous as accessible food!!! /s