Because of tampering. If an attacker can produce a pair where the public key's last 40 chars match the victim's public key last 40 chars they effectively have a

Because of tampering. If an attacker can produce a pair where the public key's last 40 chars match the victim's public key last 40 chars they effectively have a public key to dish out via MITM. How feasible it is to produce said pair is another story.