The body of each spirit was made up entirely of Life Energy.
Their bodies may have looked completely physical with flesh and bone once they gained their form but it was still made up of Life Energy.
If they took enough damage or had something that would negatively disrupt their bodies then their body would begin to break down.
If it was a far more stable Evil Spirit with more power then the effects may not have been so powerful against it.
The wood may have harmed and weakened a more powerful Spirit if it was able to enter its body but it would have to be able to break past the first layer of its body.
There were methods to harm Spirits of greater power using similar tactics but they were far less effective and had to be able to enter their body.
There were limits to how much damage could be done by using a stake or a similar tool when it was made of wood.
