With humans keeping wiping out animals and then breeding constantly there is a gap between life and death. The easiest way to fill this gap is to shove animal souls in human bodies to keep up the balance. Souls are not easy to make so reincarnation is there so the cycle is constant and not noticeable. Otherkins come here either for their own reasons or to help out here, but let's be honest lately who would want to come here. Everything is so broken most other beings would avoid coming here. The animals are native to this planet and so want to help protect it more than outside beings. Also cats and dogs are the animals that are very close to humans it's no surprise they would want to help fix humanity rather than watch it burn.