This is something I've been thinking about a lot recently. Surely when comparing inequality in societies, what we really care about is consumption. Singling out billionaires as an example of how unequal our society, I feel, is disingenuous. How much more does Bill Gates really consume than your 'average Joe'? Sure, he presumably eats good food, wears better clothing and has a big house, but his personal consumption levels are probably no more than an order of magnitude greater than ours.
What separates him then from us, considering he has a paper net worth many orders of magnitude greater than us? Well, mainly that wealth grants him an outsize influence on the economy. By virtue of his wealth and participation in the stock market he has access to some very big 'levers'. He can dictate what a large portion of the workforce choose to pursue as 'productive economic output'. He is, by means of increasing his net worth, incentivised to make prudent decisions in this regard. High performing participants in the stock market gain more influence in it. This is an optimising feedback loop for the economy and is less fair (but more effective) than simply giving everyone an equal say in the matter.
Is this a problem? Maybe. But does it really make our society more unequal? I would argue 'not as much as you might think'.