HD Wallets - Why Extend a Private Key with addition? (Non Hardened)
My question is, why is reversible arithmetic (addition of the parent private key and output from the hashing function) used instead of a “one way” function such as elliptic curve addition or others?


To allow public derivation.

This means that it is possible to give someone a parent extended key (public key + chain code), and they can derive all public keys in the chain.

If the final secret key is computed by a one-way function directly, no equivalent operations exists with public keys to compute the public version of the result.