The role of the spirit world is not to keep the physical world in balance. The two only coexist with each other. If anything, they only act and respond to issues when it affects them. For example, Hei Bai abducted villagers due to the burning of his forest by the Fire Nation. Another instance is when the ocean spirit merged with Aang in order to destroy the Fire Nation for killing Tui. The only spirit that functions to keep the world in balance is the Avatar Spirit.