Yeah, nobody wants to make the Witches angry if a determined enough one could bring down at least part of the place. [It does raise the question of whether any of them might show unfortunate colours when the surface is revealed to still be habitable, but Momo tries not to assume the worst immediately.] It looks like there's less Witches here, too? So that could be another reason. It's just a guess so far, but I've definitely spotted and talked to more Monsters just walking around.

[If their Witch population is limited, that could definitely be another reason to make sure Witches aren't going off and getting themselves hurt or killed.]