Because, simply, it's not true. I'm curious though what attack vector you're thinking of, especially one that's not related to quantum computers. Are you worried that the ECDSA public-key cryptosystem employed by Bitcoin will be broken, such that the private keys could somehow get derived easily from the public ones? If so, that still wouldn't let an attacker "transfer bitcoin from any address," since the addresses themselves are hashes of the public keys. So people would have to stop re-using addresses to receive bitcoin multiple times, since once an address has been the sender in a transaction, and its public key revealed, it would become vulnerable.