Suburbs were deliberately built to have low density to keep groups away from each other, with "inner city youths" (nonwhite people) demonized and their public services defunded. Public transit was bought out by car companies and deliberately destroyed, even leading to General Motors being convicted of conspiracy (and given a slap on the wrist). Highways were built to tear apart neighborhoods and empower suburbanites at the cost of locals, and draconian zoning laws were installed to ensure nobody could build something reasonable that could serve as a third space or impromptu hangout. All of this at a massive cost to taxpayers through subsidies and government contracts, with cities now often facing bankruptcy issues as they're unable to maintain the low density suburbs.
It takes hard work and strict government interference to make cities as inhospitable as the US'. Even just loosening up zoning laws would naturally give you cities like Japan's over time, dense and mixed-use. What real estate developer in their right mind wouldn't want to build high or medium density shops and housing as close to public transit as possible?
Food seems like it would be difficult to harvest. Anything bigger than a peach might represent a fall risk, as well as any plant that gets big enough that it could hurt someone if it gets ill, rots, and falls to the street. Better to use the inaccessible spaces for small local flora.