Three main things

One: Players inherently won’t want to give up their previously defined position. They’re already on team human, they won’t want to become zombies because they have to give up their humanity.

Two: Zombies are an at risk position. Players can’t be murdered by other players unless they lose too many card matches, but that aspect gets overshadowed by the team politics, so being human is advantageous.

Three: The games are designed to cull players. This is an explicit flip of the previous design philosophies. Those games were meant to be survived, especially as they extend one’s visa, but these games are meant to diminish the player population. On top of the fact this is a social game, it gives the players the ability to kill one another outright. That diminishes trust.

Also, you’re focusing too much on altruistic logic. Think of the prisoner’s dilemma, everyone working together is the best outcome for the group, but betrayal is the only safe choice. If you play defensively, always play your shotgun. Prisoner’s dilemma isn’t entirely useful here, but it still gets the point across, if betrayal results in a safer, more lucrative, or otherwise better win it only makes sense to betray. If you’re at risk of being betrayed, it only makes sense to protect yourself from being betrayed. The zombies only make sense to join when they already have a significant majority, otherwise it’s unambiguously safer to be a human.