Real reason why zombie rps die quickly is simple: nonody wants to put their character in danger. Zombie RP, any survival RP, Cyberpunk, Dungeon and Dragons etc. Are fictional-based RPs and the GM should be narrative. Not should, he/she must be the god of the universe. GM must be the dungeon master and create events, missions, side-missions, dangers, factions, NPCs etc. Otherwise, everyone will kill with 1-shot and will never die.

Plus, a Plague-based RP would be much better. May be in 90s, may be the Black Death, may be victorian...