Alice in Wonderland, why do they call the humans "oysters"?

I'm really uncertain what you're asking.

Do you mean in the poem "The Walrus and the Carpenter," in _Through the Looking-Glass,_ why do the the main characters ("they") talk to the oysters as though the oysters could hear and understand like human beings?

That is the premise of the poem. Walruses cannot talk as a rule either, but talking animals and even flowers are common in Carroll's two Alice books. The older oysters are wary of the Walrus's invitation to go for a walk but younger ones are eager and comply, and as a consequence they are consumed.