I believe the Tenryubito aka. World Nobles aka. gods of the modern world are, in short, really freaking tough, or at the very least, have an incredibly high constitution stat.
What birthed this hypothesis was a scene in which Doflamingo and Luffy were fighting, and Doffy says something along the lines of "i operate at a completely different frequency from you", basically a blanket implication that he, having been born a world noble, is on an indisputably higher level than any person of the world (in contrast to a Noble who is a person above the world .I don't wanna say "human" either because this world has plenty of non-human people too)
Of course, the only piece of evidence that backs my theory is the fact that in the One Piece world, Authority Equals Asskicking. (for those not familiar with tvtropes.org , I won't link you there because I have a soul. But there is an article by that name on that particular site, just saying.) We've seen it with the Ryugu royal family, King Riku and his offspring, Hell even Dalton became king and just so happens to be the toughest m'f on Drum Island. Point is, the king of any given nation is more than likely to be one of the most physically formidable characters on that island. And as we all know, The Tenryubito were founded by nineteen kings and their respective families.
And sure, 800 years gives you a lot of potential for inbreeding, but being mentally handicapped doesn't mean your body can't take a hit. It just means (if you're r**arded enough) you'll cry way more than is necessary afterward. Which is exactly what we see in Charlos. Dude can't even wipe his own nose and he took a Luffy punch square in the face, and you can not tell me Luffy held back. No gomugomu but that punch is still as bad as a point-blank shotgun blast. And during the Reverie arc he got smashed over the head with a big iron club and all he had to say was "heyyyy you hit me!"
As an additional note, the air on in Marie Geoise has to be way thinner, being on top of the Red Line. Kinda like how the Denver has Mile High Stadium and the Broncos have that advantage during home games, but i digress.; This has to has some sort of effect on their physiology, and explains the bubbledome helmets, giving them more of an explanation than "celestial dragons are so arrogant they refuse to breathe the same air as commoners" Which, I think is a great reason in the first place– really helps build the character type, but with Oda there's never just one layer.
Now I'll move onto common mythology, specifically some of the Greek myths I learned about in school, but a trope that is pretty consistent with tales of the gods. A lot of punishments that deities receive involve infinite torture. You know, like how Prometheus is still chained to a rock somewhere having his liver pecked at by vultures and regenerated daily. Follow me here-- remember when Donquixote Homing defected from the Tenryubito? His family was pursued, hanged, and tortured -- but they didn't die. The mother passed away but it was from a disease (my guess she couldn't handle the barometric pressure, see above), and Doffy had to use the Golden Gun to kill Homing and eventually Rosinante (i forget the exact details of this special pistol so I'm using a 007 reference). Anyway, there's a slim chance this lynching is a foreshadowing to what's going to happen to these people claiming such godhood. We've been given every hint necessary that Luffy's ascent to Pirate Kingship will turn the world on its head, overthrowing the immeasurably-corrupt government is a given.
If I may venture into a more speculative stance, Mjosgard wasn't going to die even if Otohime didn't save him. Those 4-5 fishmen coulda emptied their clips into him and he'd have lived. Further supporting this super-high-HP theory, he was the only one to survive a shipwreck 10km under the sea. Also, the only celestial dragon onboard, chew on that for a second. By this logic, the only reason he was pissing himself in that scene is because he didn't wanna end up like his brother(headcanon) Homing, the possibility of an eternal punishment being very real in that moment.
Pretty sure that's all I have on this theory for now, but it's still cooking and I'll probably add more as I get the chance to. Just had to write it down somewhere