One thing that seems to be common and is my working hypothesis, is that they're so smart and so respected in their field (or, in subset of cases, arrogant), that they simply trust themselves more than anybody else. There is nobody in their family and friends who can truly "slap some sense into them"; provide perspective, course-correct, nudge into objective reality or mainstream story, whatever we call it. I've actually asked a couple of them point blank - and they admitted, nope, there's nobody that they trust more than themselves. Couple of them even said - if you don't have trust and faith in yourself, you're weak.
So I'm hoping I'm somewhat inoculated against going off the deep end by having at least 3 people in my life (wife, sister, and one good friend) who can, and have, slapped some sense into me when I was going off the rails.