By the time “Day of the Dead” rolls around, “Dr. Frankenstein” recalls how basically the zombie is no longer an organism of metabolism: oxygen and food are no longer needed, as the zombie brain powers the zombie’s body. This is demonstrated with a completely eviscerated zombie which still attempts to bite the doc’s hand.
Well that was what those science guys were trying to figure out. They weren’t living in a damp smelly cave for the ambience. Problem is there is no explainations. If there was we’d spend our time poking holes through them. Let’s face it, the only answer that works is the one that my kids have for things they don’t understand. “Just 'cause, that’s why.”
Residual radiation from the space probe that exploded, maybe? Or perhaps it transformed the brains of the dead into organs that could pull energy directly out of hyperspace—causing severe brain damage and personality changes in the process—or something.
Though we should take care to differentiate between Romero zombies and Return of the Living Dead zombies—the latter of which are driven to consume human brains to nourish themselves on neural electrons. (Which also “eases the pain of being dead,” according to the zombies themselves)