This is disregarding other media, but I always figured zombies in gaming became so prevalent because their A.I. is just so easy to code compared to more sentient adversaries. If you're fighting Evil Paramilitary Organisation V-3294 they need to know how to move in a co-ordinated fashion, execute suppression fire and flanking manouevres, proper use and care of chest-high walls, etc. Zombies can just walk toward you moaning while they become a lead sponge.