This is plainly false. There are plenty of example, even recently, of billionaires losing their fortunes or going bankrupt. Often they come with criminal prosecution because they get desperate and try illegal ways to hang on to their wealth. Sam Bankman-Fried, Elizabeth Holmes, and several other examples come to mind.
There are a lot of stories of billionaires getting too risky with their investments or too concentrated in businesses and losing the majority of their wealth. The Barclay story, Jim Justice, the old Peloton CEO.
It’s not a common outcome because you have to try hard to screw up that badly when you have over a billion dollars in wealth. Parking it anywhere in common investments would leave you and your ancestors set forever.