Why don't Americans plant trees and bushes of stuff that they can eat in their houses instead of having useless grass?

Some cities actually mandate lawns. My city has code enforcement officials who have to go around and make sure that lawns are kept to a certain standard. I live in California and at some point these codes were relaxed to deal with water shortages (go figure) so we don't actually have to maintain our lawn. It's part of practices focused around preserving high housing costs (which I think are absolutely terrible).