Whether it was the monster army or the army of Martial Artists, the number of quality members wasn't that high. The Demon World had hordes of horrifying monsters, but only a fraction of them could cross over. Meanwhile, not all Martial Artists could become stronger just by having more resources.
As a result, the Transcendents and the Unrestrained could be said to be in a stalemate that was slowly tipping in humanity's favor thanks to the Heavenly Strength Society's contribution. However, the rest of the army wasn't doing so well.
The strongest Martial Artists could intercept weaker monsters and kill them faster before they pass them, but scattering their attention to not let anything pass them was no different from asking for a quick death. The responsibility to clear the weaker monsters had to be distributed in accordance.