They're well evolved to survive in social groups. Even if they suspect someone is recruiting for something terrible, the loss of social status and allies isn't worth confronting the recruiter. Even visibly suspecting it will harm social status. This is a feature, not a bug.

Historically, the kid who called out the emperor's new clothes was executed. Every time you started shit in your tribe, you risked being shunned, which meant death. If the whole tribe was crazy, you were better off being crazy too.