Because he may be very clever at having come up with this whole ethereum system, but that doesn't mean he's wise and experienced. It's a rookie mistake, made by a rookie. It happens. He seems the type to learn from it at least.
It is not like gpg where the private key is independent, stored on disk and encrypted by the passphrase.
The user messed up in that he didn't understand that he needed to use a strong passphrase or it was possible for anyone in the world to guess it.
Are you saying that is a good thing? It's well known that brainwallet is a footgun.
https://en.bitcoin.it/wiki/Brainwallet
>An early old-style brainwallet was created by by memorization of a passphrase and converting it a private key with a hashing or key derivation algorithm (example: SHA256). That private key is then used to compute a Bitcoin address. This method was found to be very insecure and should not be used. Humans are not a good source of entropy.