Because the army felt allowing the crews to name their aircraft and apply nose-art was a morale booster.
While the Germans were a bit more conservative, they allowed thier crews to do the same.
This is actually a tradition that goes back to WWI, where aircraft not only had names and art applied, but in many cases, the entire aircraft was painted to the pilot's liking.
By the way, "Aluminum Overcast" is a B-17G.