Because it's almost certainly false. The source for this is Bruce Blair, who while is a credible source on matters of nuclear armament, would not have been privy to this information.
The DoD has refuted the claim made by Dr. Blair also insisting that he was never in a position or authorized to know these codes. Furthermore, the House Armed Services Committee which investigated this matter was unable to find anyone else to corroborate the claim. If Dr. Blair's claims were true, there would certainly be one other officer (retired by now) who would be able to confirm it as part of a Congressional investigation, and yet everyone who was in a position to know this has said the claim is false.
The DoD has refuted the claim made by Dr. Blair also insisting that he was never in a position or authorized to know these codes. Furthermore, the House Armed Services Committee which investigated this matter was unable to find anyone else to corroborate the claim. If Dr. Blair's claims were true, there would certainly be one other officer (retired by now) who would be able to confirm it as part of a Congressional investigation, and yet everyone who was in a position to know this has said the claim is false.