Human beings are the only living beings to consume a whole list of food products that aren't possible without modern processing (cooking, fermenting, pickling, etc) and we eat many "normal" foods in ways that other animals wouldn't. Why isn't really an easy question to answer other than to say because we decide to. This is a very common vegan talking point but I don't think it's a particuarly poignant point to make from that perspective (I don't know if that was your intention). It's sort of like asking why we eat bread, because no other being consumes anything remotely similar.