Let me start with something you might dismiss as trite - Correlation does not mean Causation.
A money manager charging say, 1%, isn't likely to take on clients below a minimum level.
On the other hand, there's a long debate regarding how, on average, managed funds don't beat the averages.
I think that you should look at it this way. People that have money tend to be focused on other things. A brain surgeon making $500K/yr may not have the time, nor the inclination to want to manage her own money.
I was always a numbers person. I marveled at the difference between raising 1.1 to the 40th power, getting 45.3 (i.e. Getting 45.3 times your investment after 40 years at 10%) vs 31.4 at 9%. That 1% difference feels like nothing, but after a lifetime, 1/3 of your money has been skimmed off the top. the data show that one can do better by simply putting their money into a mix of S&P index and cash, and beat the average money manager over time, regardless of convoluted 12 asset class allocations.
Similarly - There are people who use a 'tax guy.' In quotes because I mean this as an individual whom they go to, year after year, not a storefront. My inlaws used to go to one, and I was curious what they got for their money. Each year he sent them a form. 3 pages they needed to fill in. Every cell made its way into the guy's tax program. The last year, I went with them to pick up the tax return. I asked him if he noticed that they might benefit from small Roth conversions each year, or by making some of their IRA RMD directly to charity. He kindly told me "That's not what we do here" and whisked us away. I planned both questions in advance. The Roth conversion was a strategy that one could agree made sense or dismiss as convoluted for some clients. But. The RMD issue was very different. They didn't have enough Schedule A deductions to itemize. Therefore the $3000 they donated each year wasn't impacting their return. By donating directly from their IRAs, this money would avoid tax. It would have saved them more than the cost of the tax guy, who charged a hefty fee, in my opinion. It seemed to me, this particular strategy should be obvious to one whose business is preparing returns.