The reason why people have not simply picked up and rebuilt the ruined cities or lived in them is because Gnormats swarm around them, knowing very well that people and food are most likely to be around. When entering a city, you are bound to see many different Gnormats around. Fighting, eating eachother or other people, mating, tearing things apart. When enterting a large city ruin one must take caution, make sure that none sees you or finds evidence of you or you will be hunted down.