There was one article I read in kotaku.com (I think). It was a discussion about the why are game developers making most survivor games into a fight for survival from raiders, zombies, creepers, and any hostile creature that means to take the player out of the game. It continued on about a survival game isn't just about defending from enemies or taking out any competitors. Survival games are about living in harsh environments, removing yourself from your home and managing to stay alive.
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Despite the good argument, having enemies that attack periodically can be exciting and fun. Like Left for Dead and Dead Island where players have fight off zombie hordes to reach their goals by either gunning down zombies or cutting through with axes, hammer or whatnot melee weapons.
I found the article enlightening and found myself appreciating the game mechanics of a survival game. I have played Banished, Left for Dead and Dead Island and they are all great games to play. Its really about what people like to play and shoot zombies is way easier than to manage a town.