so the real issue is why we would need to invent a new rather obscure algorithm to do a brainwallet when we could achieve the same thing by just increasing the length of our passphrase
Every existing "classic" brainwallet is attacked by billions if not trillions of password hacking attempts per second. See
Collection of 18.509 found and used Brainwallets. By adding BIP38 to the equation, suddenly an attacker would only be able to do a few attempts per second. It's not worth the electricity to even try.