Even the wooden artifacts inside are being rebuilt. I was in Ise watching the rebuild the previous time. Bridges, buildings, everything. The new building is built beside the old building, a new bridge beside the old one and then you tear down the old ones (and that material is then re-used at other smaller shrines). Next time you swap back. I was wondering about this system until someone in an Ise pub handed me a leaflet where it was all explained. The Japanese believe that what's important to keep is the knowledge and the craft - how to build the things. The actual buildings aren't important - the history is in the craft, not in the buildings themselves. If nobody keeps building then the "how" is forgotten and the historical buildings will then just degrade and disappear over time anyway. And I understand that completely. I've watched how very experienced people try to recreate the techniques of building old boats, and even after years and believing they "got it right" there are still gotchas (and near-fatal disasters). So the Japanese figure that rebuilding everything every 20 years is just enough to transfer knowledge and the "how to" to younger people. A good millennium of rebuilding is a good track record in that respect.