Picketty's key point is that "the rate of return on capital (r) is greater than the rate of economic growth (g) over the long term."

"What Piketty finds is that in all developed countries the wealth:income [GDP] ratio is high and rising...

"Piketty's theoretical contribution is to argue that we should take these empirical findings at face value. The way capitalism works, says Piketty, is that existing wealth earns a 5 percent rate of return, r. The total pool of labor income, meanwhile, grows at the rate of overall GDP, g. When r is larger than g the pool of wealth owned by wealth-owners grows faster than the pool of labor income earned by workers."

Those whose money is obtained as income rather than as wealth from capital don't invest much in the stock market. So private fortunes of billions of dollars are based in part on this hidden advantage that Picketty uncovered.

https://en.wikipedia.org/wiki/Capital_in_the_Twenty-First_Century

https://www.vox.com/2014/4/8/5592198/the-short-guide-to-capital-in-the-21st-century