The odds of disaster are >10%, if history is any indicator. We had two world wars in the last century, for example. Events happen. Past doesn't predict future, but there are dozens things which can go wrong.
If I had $100B, it would make sense, from purely an expected value perspective, to invest up to perhaps $10B preparing. An expected value perspective isn't the right way of looking at this, but it's hard to argue that a <$1B investment doesn't make sense. That's 1% of an ultrawealthy person's wealth, and almost not noticeable to lifestyle.
The ultra-prepper lifestyle is irrational, but it's the rich person version of having a stock of non-perishable emergency food and medicine in the basement. If I had $100B, and didn't care about other people, I'd do the same.
Footnote: spending $1B on prepping is enough to change the lives of a million of the very poor. That's 3 years income for people making $3/day times a million people.