It's pretty obvious why zombies became what they became. It's not just it being an artistic trend or whatever, it's moreso due to the fact that they're an easy replacement for the average videogame baddy. If you want to make any "mature" realistic game filled with gore and killing people, but you don't want the actual killing people part to raise eyebrows, just make them zombies and voila, there's the generic basic enemy you can use for your entire game.
I would have said they are a general substitute for any decent enemy AI. A regular mook AI still has to display some inteligence and tactics to be able to pose a threat, a zombie AI needs to charge at you and that's it.
You know it deserves the GOTY.