I think it's a combination of the effort required and sadly the liability too. I would imagine anyone who is saying "feel free to come eat this food" is exposing themselves to lawsuits, to some degree. The kinds of organizations who are large enough to make a big impact by deciding to grow some food on their properties are the same ones who'd be targeted by frivolous lawsuits, costing money just to defend against, and offering the orgs no tangible benefit in return.

To be clear, I don't agree with structuring things this way and I think it's a trash way for our society to work, but growing food in "public" places seems non-viable without addressing that big vulnerability somehow.