Low Efficiency
Even if the wood splinters were from just normal trees, it didn't mean that they had no wood element in them.
All things in the world contained energy and had an inclination towards one or more elements. Just because one couldn't see it or make use of that energy didn't mean that it didn't exist.
Of course, even for cultivators, refining this energy from mortal materials was not a viable option. They might be able to extract some energy, but the effort and time needed to do that would not be worth it. And even that tiny amount would be extracted from a massive amount of raw material.
It was the same as using hundreds of kilograms of rose petals, just to extract a few milliliters of pure rose essence oil.
But in the case with Lin Mu, he didn't need to put in as much effort.
Omnicore Ascendancy directly broke down materials into their base elements, thus Lin Mu wouldn't be spending too much time in doing it.