If the forensic examiner is allowed to "guess" what's on the drive, why can't the owner of the drive "forget" the passphrase? I have an encrypted filesystem fr

If the forensic examiner is allowed to "guess" what's on the drive, why can't the owner of the drive "forget" the passphrase? I have an encrypted filesystem from years ago that I've forgotten the password to, it has some vacation pictures on it that I'd like to retrieve, and every once in a while I try to guess the passphrase but so far I've been unable to.