People should read how the Forbes 400 list got started. It was extremely difficult to get off the ground. It's hard to know who has the most wealth -- it isn't public information, after all. Those who become wealthy via public markets have their ownership disclosed, so we can place a minimum value on their assets, and as a proxy, their total net worth.

The changes to the list over the decades could be more a function of wealth disclosure, which are mandated by the regulatory requirements of equity markets (e.g., we can all look up Zuckerberg's percentage ownership of Facebook).