Seth is not attacked by zombies because it is a prevalent theory amongst fans that his glass eye grants him immunity to attacks from the undead.
This widely discussed fan theory suggests that Seth's distinctive glass eye somehow renders him immune to aggression from reanimated corpses. While the game's narrative does not explicitly state the reason, the community has embraced the idea that this unique physical trait prevents the undead from perceiving him as a threat or target. This has become the assumed reason within the fanbase for why he remains unmolested by zombies.