I wonder every time I watch one.

Seriously, people in vampire movies have heard of vampires! In alien invasion movies, they land and everyone says: "OMG, Aliens!" But in every zombie movie the dumb girl is always turning to the tough guy saying: "What the Hell are those things?!" When they're clearly zombies.

There are exceptions, of course, like WWZ, but I'm talking about the major, traditional classics, i.e. Day of the Dead, Dawn of the Dead, etc.

Because people in zombies movie general have an I.Q. of 4.

*Generally

It's because of this thing called genre blindness. Zombie movies are usually set in alternate realities where the Zombie genre doesn't exist, which is why, in The Walking Dead, the characters call the zombies "Biters", "Lamebrains", "Geeks", or, most notably "Walkers". Of course, Glenn mentions portal when he's about to be lowered into the well, but speculation about this would take a lot (as in a LLLLLLLLOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT) of time...