The soul was a mysterious thing, and the majority of powers and sects across the world knew very little about it, only having a scarce few techniques that directly related to the soul that they didn't understand, but practiced as inheritance from generation from generation.
To further emphasise the point of how special the soul was, it had to be recalled that although the Nascent Soul stage was a high stage of cultivation that few would reach in their lives, it was also the one that cultivators stalled at. Most cultivators who ascended to a high stage would spend the majority of their lives in the Nascent Soul Stage, not daring to break through to the Resonanace Stage, as one would need confidence not only to pass Resonance but also into Void Returning. It was scary, and it was unstable. The life span of a Nascent Soul Stage expert was already very long, and advancing to the Resonance Stage didn't increase that lifespan at all.
