Laws.

In the same way that you said you could see it happening in favellas and slums. In those places in non 1st world countries the rules are so unenforced its probably easier to get access to fresh food because all you need is some guy with a pickup truck to.drive to the market every day, load up, drive back and set up a stall.

Doing anything permanent or even semi permanent in a 1st world country involves insurances, permits, vehicle inspections, taxes, zoning, city council...