No one has actually seen the witch do anything, they just assume. And the creatures have not been seen in a long time, grandmothers might have memories of seeing them when they were young, but they might also be senile.... The offerings are habit, part of the Old ways that run deep into their bones. Belief in the old ways isn't a requirement by the way, that's just painting how the village is. If you want your character to chafe at the old ways or wonder what's beyond the village or wonder if the Crows aka Priests of the White God have the right of it, then feel free.