OK, let me throw a hypothetical at the Kmart shoppers.

If you were hungry and you saw an apple tree loaded with ripe apples, even windfall apples on the ground, would you eat some?
If you were facing a food shortage in your house would you fill your pockets with them and give some to your wife?
Would you get a shopping cart and load it up with them so you could feed your family and the neighbour too?
What about a landscaping planter that the city parks department had full of ornamental kale?

Are wild and wasteland plants seasonal?
Yes, you bet they are, and opportunistic too.

You are not going to find greens in the middle of a parking lot or in the middle of winter. If you open your eyes and go for walks you will start to see much more than you expect though.

As for the complaint about identifying them being hard, well get a good book on plants and take a senior citizen out for a stroll.

One last hint, most of the plants you will find interesting as food plants inside cities are cultivated plants or escapees from cultivation, and your worst competitors for them are squirrels, racoons and birds.