More volume than any of their characters.
Look at those clean, voluminous tresses! In real life, long hair that's left to dry naturally without product will usually either hang limp against the scalp like depressed pasta or turn into a frizzy head-labyrinth. Nothing in nature produces those soft, perfectly symmetrical curls. They're the work of either a blow dryer and a round brush or a curling iron and a hell of a lot of mousse, and none of those things can be grown in someone's survivalist vegetable garden or rigged together out of the corpses of our new robotic overlords. You can get a similar look by wrapping your hair in rags or plastic curlers and leaving it like that overnight, but we see these women at night, and none of them is sporting a 1950s-style curlers-and-headscarf ensemble.
A zombie-apocalypse scene judged too shocking for viewers.
The post-apocalyptic phantom hair product problem has been going on for a long time. When Planet of the Apes came out in 1968, the female lead had '60s-style helmet hair that we were meant to believe had just grown that way. Twenty years later, post-apocalyptic women were suddenly blessed with feathery '80s dos. Of course, no one expects movies to be 100 percent realistic when it comes to appearances, but why do shows and movies that pride themselves on being gritty and realistic make everything gritty and realistic except women's hair? Is there a tough union of hair stylists that threatens to break directors' legs if they don't get enough working hours? Or do male costume designers assume that female hair is naturally that pretty and that loud gun-shaped machine in the bathroom is just what women use to blow their boobs into shape every morning?
The human female, seen here in its larval "six in the morning" stage.