It's called "the Romero Effect." In the world of zombie movies, George A. Romero does not exist. He is the man who has been credited as the father of zombie movies. So, with him not existing, everyone in those movies have no idea what's going on and thats why they are referred to as " walkers" or "undead" because the word zombie has yet to be coined. There are a few exceptions to the rule such as Warm Bodies. In that book/film Romero exists because there are zombie books hidden throughout the movie and the undead are actuallu referred to as zombies.
They HAVE heard of zombies in "Shaun of the Dead". Shaun just doesn't want to call them that.
"Don't say that!" "What?" "The Z word, don't say it." "Why?" "Because it's ridiculous."
It's called genre blindness. It's generally necessary because if you accept that everyone in the movie is familiar with the genre of movie they're in there would necessarily have to be self-referential dialogue about the genre, which is usually only appropriate in comedy.
It's the same reason why everyone in horror movies constantly make retarded choices.
Actually it got explained in walking dead by the director that it takes place in an alternate universe in which there have never been any zombie movies or anything of the such taken place.
The "alternat universe" is a good explanation. In that universe, there also has to be suspension of the first and second laws of thermodynamics, because the zombies move around (expending energy) without taking in any energy (until they eat a person, but most of them would be immobile by then). Also, after a year or two, why aren't they dried out?
"Don't say that!" "What?" "The Z word, don't say it." "Why?" "Because it's ridiculous."
It's the same reason why everyone in horror movies constantly make retarded choices.