There are many, many ways that wealth is realized without spending it. Once you get past a few hundred million you literally cannot spend the money as fast as it comes in.
These billionaires aren't amassing wealth to spend it on things, they're gathering it for power, leverage, status, prestige, legacy reasons etc, and using it to influence and control the world to further their own agenda.
In Bezos's case, he built a company that literally didn't even exist 27 years ago. It's gotten so huge, because globalization has gotten so effective. That's true of all tech companies: they benefit disproportionately from globalization and network effects.
As far as what to do about it? The only solution could be to break up Amazon, Google, Apple, etc. Taxation is not it. The Federal government and the Fed controls the money printer and influences how high these stock valuations are getting. They're a large part of the problem.
We can't print trillions of dollars without side effects in supply, demand and price mechanics. In this case, dollars are chasing productive growth assets as yields in bonds hit the floor and go under zero in some countries.
The growth in inequality correlates with the growth in the national debt, and monetary supply growth.
These billionaires aren't amassing wealth to spend it on things, they're gathering it for power, leverage, status, prestige, legacy reasons etc, and using it to influence and control the world to further their own agenda.