Because no one who would be interested in disproving it trusts the person offering the bounty to be doing so in good faith.
Credibility? You have to be joking, what does he actually have to lose? According to Wikipedia he had a quarter of a billion dollars 15 years ago! He gets to live by rules you and I don't even know are possible, but suffice it to say that credibility is only ever a problem for the little people. Look at trump, after all.
Sure, Just like Elon and Zuck he'll couch his ego trips as for the people and pursued at great personal cost, but what actual price is failure to him? I'm guessing his true risk is limited to some teasing in the sauna.
I think of that old joke:
>>>>
The young economist looks down and sees a $20 bill on the street and says, "Hey, look a twenty-dollar bill!"
Without even looking, his older and wiser colleague replies, "Nonsense. If there had been a twenty-dollar lying on the street, someone would have already picked it up by now."
>>>>>
Surely there is not a $1M bill lying on the ground, ripe for the taking. That's why I'm not bothering to claim it.
He provided a 30 page document referencing the analysis of other doctors, explaining the rationale for those estimates. He has further offered $1,000,000 dollars to anyone who can show that the estimate is off by a factor of 4X, i.e. making the task easier for the person challenging his extraordinary claims.
His claims are so far from anyone's expectations that it should be trivial to disprove, right? There's a bug bounty of $1M dollars available to anyone. Why is no one bothering to claim it?