There's a real feeling out there that civilization has become a machine that is running out of control. This is certainly not a new feeling. For millennia, people have commented on how society is either inefficient, bloated, and stifling, or downright malicious towards some of its members. After all, the old growth has been hacked away, leaving room for the new. Life gains purpose and reason when your boring job is replaced with a savage fight for survival.
People fend for themselves or band together to further life.
People usually become healthier, too, at least those who survive long enough, and not by throwing away calories at a gym. It makes a nice read from a soft, warm, couch. A good post-apocalyptic book combines this newness with a crushing bleakness. Everywhere one looks, one sees death and destruction. There is neither escape nor hope for a quick return to the old world, and an end to the struggle, if it exists, is far off.