Capital gains are only taxed when they’re realized. They don’t pay taxes when the value of their portfolios go up, just when they cash out. So instead they borrow against those portfolios and pay zero taxes.

Estate taxes are supposed to close this loop.

edit: Pokemon Cards are my go to analogy for capital gains. You can’t tax someone on how much their pokemon card collection is potentially worth on ebay. You can only tax the money they made from the sale.