Those wealth management companies charge significant fees and typically do not beat the market once fees and legal expenses are considered.
If you look at any list of the truly rich, it is almost all 'new money', of those that are beneficiaries, the money was inherited relatively recently (such as the Walton family).
The goal of a wealth management company (targeted at the very wealthy) isn't just "I want to beat the market". A wealthy enough person can live a comfortable life on savings account interest and still be wealthy when they die. The goals end up being a) beat inflation enough to cover my expenses and b) protect from any risk of not being wealthy".
There are quite a few problems with the Forbes 400 list, most notably it does not in fact capture the wealth of the 400 wealthiest people/families. A lot of old money is left off these lists because...well it's old money and they don't want the publicity.
There are a bunch of other things to consider, for instance diversification of wealth has fundamentally transformed since the early 1900's and wealth management is on a completely different level than even 20 years ago, let alone 120 years ago. Not to mention the legal innovation to protect estate value that has been going on.
I'll also point out that the people on that list are generally not using Fidelity, Vanguard, or Hancock as their wealth management services. They are using private family offices or firms like PDT Partners that are directly accountable to those families.
If you look at any list of the truly rich, it is almost all 'new money', of those that are beneficiaries, the money was inherited relatively recently (such as the Walton family).
https://www.forbes.com/forbes-400/