Wood can handle the occasional wetting much better than most other building materials.
The wood inside that concrete won't rot because you need more than moisture for wood to rot. You also need oxygen. There may be some oxygen inside the concrete and the wood cells, but once that's used up, the very slow rate at which oxygen could diffuse through the concrete and wood to get to the site where the wood is rotting would effectively stop the rate of wood rot.
It's that additional need for oxygen that's the reason why 200 year old logs can be brought up from river bottoms and still be in near perfect condition. There's lotsa moisture on a river bottom, but not enough oxygen to support the rotting of those old logs.