I'm guessing they just couldn't risk turning goody boy Spiderman into a zombie...
Plot of comics What if... Zombies
Within the Marvel Multiverse is an alternate Earth designated Earth-2149, which contains alternate zombie versions of Marvel superheroes. The story begins as an unknown superhero from another dimension, brought by the "Hunger", crash lands before infecting the Avengers, X-Men, Fantastic Four and almost all other superheroes. The infection spreads by contact with the blood of the victim, usually through a bite by an infected individual. The zombie super heroes largely retain their intellect, personality and their super powers, although they are consistently driven by the "Hunger" for human flesh. (Captain America notes that he is still functioning with -literally- half a brain, having been partially decapitated by Magneto with his own shield.)[4] As the story begins, the Marvel Zombies have already devoured all non-transhuman human life on Earth and are now without food (as prefigured in Marvel Zombies: Dead Days, a later tie-in).
The series begins where the Crossover story-arc ended, with Magneto destroying the cross-dimensional transporter after the Ultimate Fantastic Four and survivors escaped back to the Ultimate universe. After a battle with the zombies, Magneto learns that the Acolytes and Forge are still alive on Asteroid M. However, before he can reach them, he is attacked once again, killed and eaten by the zombies, though he manages to behead the zombified Hawkeye. The Silver Surfer arrives on Earth and informs the zombies that his master Galactus is en route to devour the planet. The zombies attack the Silver Surfer, who is overpowered and devoured by several of the former superheroes: Colonel America, Iron Man, Giant-Man, Spider-Man, Luke Cage, the Hulk, and Wolverine. After acquiring a portion of the Power Cosmic, they slaughter most of the remaining zombies, intent on satiating their hunger despite the latters' unpalatability.
The Acolytes return to Earth to find Magneto, but instead discover a still-living Black Panther. The Panther has escaped from the lab of the zombie Giant-Man, who has been keeping him alive as a food source, as a result of which the Panther is now missing an arm and a foot. Zombie Wasp gets into an argument with her husband when she discovers he was hoarding the Black Panther. He promptly decapitates Zombie Wasp by biting off her head, which remains sentient. After observing the Wasp's head begging for flesh, Black Panther reasons that the hunger is more psychological than physical.
Meanwhile, the zombies have decided that the flesh of other zombies just isn't satisfying. Galactus then arrives on Earth and is attacked by the zombies, but he repels them easily. Giant-Man, Iron Man and Bruce Banner create a device that amplifies the powers they gained from the Silver Surfer, and together with Colonel America, Luke Cage, Spider-Man and Wolverine they are able to injure Galactus. The cosmic-powered zombies fight off zombified versions of several supervillains, although Colonel America is killed by the Red Skull, and then proceed to devour Galactus. Giant-Man, the Hulk, Iron Man, Luke Cage, Spider-Man and Wolverine are then infused with Galactus' power cosmic' the group thus becomes collective Galacti.
Five years later, Black Panther, the Acolytes, and the Wasp, restored with a cybernetic body, return to Earth to find the planet depopulated. Unknown to them (although it is one hypothesis), the Zombie Galacti have taken their hunger to the stars.
In the final scenes, an intelligent alien race on a distant planet is fearful of the coming of Galactus, as they can see the signs of his imminent arrival in the night sky. To their horror, the Zombie Galacti land on their world instead and immediately begin to consume the populace.