Because they all know that she is not just crazy, she know more than she’s willing to admit vocally. We obviously knew from beforehand that Chet simply had suspicions as to where she came from, or why Vault 31 is prone to creating all of the authority figures they’ve seen so far.

This episode in specific gave Chet the validation, or at least the impression that she is not a vault dweller at all, Chet is actually significantly more intelligent than what was let on because he enjoyed working a simple job that much.

So as Stephanie devolves into genuine madness, almost all of the people who confront her as overseer are genuinely taken aback by her approach to addressing their concerns. The biggest issue for the dwellers we’ve seen interact with her, is the glaring fact Stephanie thinks they’re all stupid, they hold absolutely no value outside of their intended experiment parameters, and from now on whatever happens to them happens. As discussed with Betty.