They are there to make sure none unauthorised launches the nukes,yes. But there is a chance someone within the military is bought by someone and that adversary doesn’t even have to be the official government of a foreign country.
To give a few examples(even though the US nuclear policy has changed and it wouldn’t be possible today, thankfully): What if Putin, with his back against the wall, decides to risk it all and by proxy let the US attack China so NATO won’t come after him?
What if Winnie Pooh faces a revolution and decides in a hitleresque manner that it China is no longer under CCP rule there better is no China and orders a loyal sleeper to attack China so there at least is a chance that he comes out of the bunker irradiated but victorious?
We all have seen enough crazy shit to not rule out even more crazy shit.
It was just a bad, lazy, process. nothing more. And I really hope that the US really did change it in the meantime.