The Mona Lisa was taken to France by the painter himself and sold to the king by his heir. Why should they "return" it? Why doesn't Clooney focus on things that were really stolen, such as the Veronese that faces the Mona Lisa in the Louvre, or the portrait of the Emperor stolen from Prague by the Swedish Army:

He could also ask Milan to return the paintings that were stolen by the French from other places only to be "returned" to Milan. For example, Città di Castello: