Why does Siduri not believe Gilgamesh is who he says he is? Cite evidence to support your answer

When Gilgamesh tells Siduri who he is, Siduri asks him why he looks the way he does if he is indeed the great Gilgamesh.

Then Siduri said to him, ’If you are that Gilgamesh who seized and killed the Bull of Heaven, who killed the watchman of the cedar forest, who overthrew Humbaba that lived in the forest, and killed the lions in the passes of the mountain, why are your cheeks so starved and why is your face so drawn? Why is despair in your heart and your face like the face of one who has made a long journey? Yes, why is your face burned from heat and cold, and why do you come here wandering over the pastures in search of the wind?