Why does Shakespeare list the ingredients in Macbeth?

'Cause it's FUN! The recipe is extremely entertaining. Also, the ingredients are mostly pretty gross, if not poisonous, so if Macbeth drinks them (which he does in some productions) the audience goes "eeeew!". Also it has been pointed out that some of the ingredients are from animals which are ambivalent, frogs and newts which are not sure if they are land or water creatures, or bats which fly like birds but have wool like mammals.But mostly, it's fun.