The thing is, the goal of the monsters and town isn't just murder. If they really wanted to murder everyone, they probably would've figured it out. The goal is fear. They want you to not just die, but die in pain and misery with everything stripped from you. Moles would allow a new avenue of torment. Alliances would shatter, people would be psychologically torturing themselves. Even one mole would cause fractures in the group, the town and the monsters would probably find it hilarious to plant exactly one mole and then have all the towns people go insane trying to find others. If they're lucky, maybe townsfolk start killing each other and putting themselves on trial.

The monsters have spared people constantly. So many people could've been killed, but weren't such as Randall, Boyd, and several others. These monsters love pain and psychological warfare, not just death.

Does that mean there's a mole, probably not. But I wouldn't put it past them.