why is it that an exact replica of art is valued less than the original even though the aesthetic aspects are still the same?

One obvious difference is that the original is 'scarcer' and thus the laws of supply and demand lead it to become 'more valued'. Also, the original has at least one potentially valuable attribute the replica does not have, the attribute of being 'made by the original artist.' Finally, it seems to me that most appreciators of art will deny your main premise... the claim that there is such a thing as an 'exact replica' of art.