Because the soul reincarnates into bodies, and the soul is immortal and only created once. Then we have many lifetimes and the soul evolves from different experiences and scenarios.

Thats how the theory goes at least. And why do we do this? They say we are all parts of one being experiencing his creation through us, humans, what we see daily.

Its just what they say. No idea if it’s true. But I like interesting theories.