You're not seeing the whole picture.

Charles Koch's Industries company has no shares. Bill Gates' farms have no shares. Bricks of gold, have no shares. Jeff Bezos, every other billionaire, and soon to be Elon Musk, are diversifying publicly traded shares of their own companies, to other assets.

If markets net worth gets taxed and nothing else does, then billionaires will take their money out of markets altogether and the market will crash. Which might sound all revolutionary and fun but it would totally kill our economy.