Unless you are prepared to present evidence that in the middle ages oak was never used for construction of sturdy objects, this line of thought is tinged by modern bias.
Edit: OK, so I went ahead and checked it myself. Oak was used for reinforced constructions that would be subject to major stress such as fermentation barrels (including metal bands with rivets) and ships. It was also an expensive wood used for flooring and paneling. As such, it might be that oak is the wrong wood to use here because of the cost (an entire massive box made out of it would be quite expensive), but on the other hand it fits the use it was put to in the comic perfectly.
Given that other woods do not significantly save on weight, I suppose we could change to pine or pruce. Does the DM manual say anything about materials used in reinforced doors?