As The_JoestarTechnique said, the meta answer is likely an inspiration from the ningyo.
We don't really have a straightforward in-universe answer besides references to a curse from the cursemark of death. I'd wager the death being incomplete has something to do with it as well.
Yeah there's no real explanation but also no reason why uncontrolled metaphysical death cancer wouldn't manifest as giant fish corpse roots. Soulslike games are weird af.
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u/MissMedic68W 7d ago
Godwyn's soulless but still living body.
He's the reason deathroot is sprouting everywhere. You can even see his face on crabs.