I think Excidium said it best, zombies are rather a force of nature in these movies. They're closer to the radiation in Fallout, or the various wildlife you fight in common RPGs.
It's not the zombies themselves why people hate that entire genre of movies and games, it's mostly everything that comes with that.
First, the setting is almost always contemporary and urban. That was the selling point, the idea that you take the fight right down in your neighborhood, fighting dead versions of your friends in your favorite mall and then beating some random douche in the head with a baseball bat. You fight with contemporary weapons against contemporary enemies. It's all about the scavenging and surviving but at the end of the day, if you don't have your fireballs or your uber-armor, or some laser weapon, you're not gonna draw the attention to fantasy/sci-fi nerds. History nerds obviously don't care about this shizzle, so you're basically left with a realistic setting that only normal people that never fought/saw enough zombies care about.
Second, the setting is almost always apocalyptic or post-apocalyptic. As in, zombie movies can be referred as disaster movies. And while that's okay, the general story in itself is overly dramatic, not very adventure-ish and a bit boring. This is also the reason why most people like Mad Max 2 over the first one, or why Fallout is a great setting. Because they're not "post-apocalyptic" as much as they're "post-post-apocalyptic". They're not about dealing with the apocalypse or surviving the impact, but rather picking up the pieces and living in a different world. The pretty past is no longer the subject of discussion anymore (or people never managed to experience it), strong societies are being formed and so on. If Fallout was just about surviving in the wasteland, people would pretty hate it. But as a spaghetti western set in the post apocalyptic world and an adventure filled with colorful established factions and new threats? It really works.
Some zombie stories do try to do a post-post-apocalyptic thing once in a while, like what The Walking Dead went halfway through the comic.
Third, the fact that you mostly just have zombies and leave it at that. Again, it's been almost forced on the zombie genre to never really reveal what started the thing, what exactly is the cause of the outbreak, if a cure has been found or other stuff. Zombies are almost always straight-forward enemies, and that's it. For grognards who are used to various types of demons/aliens/dragons/elves and so on, this shit ain't enough. Ironically, it was this shitty Twilight clone movie that tried to do something fairly unique with zombies, by curing them with love. Every other movie though just ends abruptly, mostly with the main cast dying. I want to see people using zombies as weaponry, exploring more of their weaknesses than just SHOOT DA HEAD BRO, maybe some frankenstein zombie specimen made by sewing different zombies together.
So basically that's what's wrong with zombies. Not the zombies themselves, but everything else about the setting that just doesn't lend itself well to adventure-type of stories one is looking for fantasy movies and games.
