Art forgery/reproduction is a well studied craft and as such making decent replicas is not hard (my wife has a half dozen fairly decent Monet's and Matisse's kicking about our attic from her art school days). But even if you could reproduce a painting to the point where it is indistinguishable from the original it still doesn't reflect the fact that paintings are historical artifacts as much as simply an aesthetically beautiful objects. And much like any historical artifact much of its value lies in the fact that you are looking at an object that was "there" with "them" when "it" happened. That is something you can never reproduce.