Creator Robert Kirkman has previously said that The Walking Dead universe is an alternate reality where there were no George Romero-type zombie movies, so the characters have no frame of reference with what they are seeing.
Well, it’s his story but it bothers me, too. We’re suppose to relate to these characters and wonder what we would do in their place, but we’ll just say that they’ve never heard of a zombie? I don’t know, that’s just weird! I hadn’t heard that they didn’t know about zombies, I just assumed that they didn’t think zombies could ever be a real life thing so when confronted with what would be called a zombie, their thoughts just didn’t go there because that wasn’t real life. But another thing that has kind of bothered me about TWD is that sometimes they’re over prepared and sometimes under. Take the season at the farm and even before— people sleeping in tents? Hell no when there are cars, a Winnebago, or a house around! And why did they never board up the farmhouse windows? It never made sense to me. Also, later on, why not double back to the farm? You knew what you had there, it was isolated, and the herd may have moved on.