The companies - both public and private - that can contain or produce that level of wealth are very well known and almost impossible to hide. It's rare for Forbes or Bloomberg to find new stray billionaires that had been hiding for a long time. It does happen [1], it's just quite rare, especially versus the size of the known list. You don't magically quietly generate or inherit billions in wealth, you move things when you do that and it makes headlines (either over time or when a liquidity event occurs), it garners a lot of attention.

[1] https://www.forbes.com/sites/maddieberg/2019/02/19/the-great...

here is the full list of the forbes 400: https://www.forbes.com/forbes-400/