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There are many different grades of aluminum. Some oxidize readily and others hold up quite well to the elements. You can find aluminum boats & canoes that are 50 years old and still in great condition. You can also find a 2 year old beer can that is eaten up from moisture. It all has to do with the alloys.

Larry, you are right on that aluminum will act as an anode when touching steel. Likely the factory did put something, like tar paper, as an insulator. The insulator probably deteriorated over time allowing the galvanic action. Dissimilar metals will always have some form of galvanic corrosion on one of the metal types.