One thing I don't understand is how the hell the kingdom has been standing for so long. If everything relies on Spirit power and they mistreat the spirits why do the spirits stay? As a comparison it'd be like if electricity was alive and we decided to torture it for fun and it had the ability to just leave.
I agree given the tone of the story, but it may be akin to an abusive relationship. A lot of people found it difficult to leave their abusers due to sunk cost or other, more sentimental reasons—even just "occasional kindness" was sometimes enough to keep someone reeled in.
Others rely on terror and gaslighting, like being told that the abused won't find acceptance and love elsewhere. The spirits were told that the Hero would be their savior figure, for example, so they should stick around for the Hero. Others were simply blackmailed, like the sea spirits here, being promised with various lies.
Others were just simply too tired to even attempt to fight back. There's this neat little thing called learned helplessness in psychology—show someone enough times that there's no way out of a hopeless situation, and they stop trying to fight or avoid it.
That said, again given the tone of the work, I doubt the author put that much thought into this, lol.
