If you can borrow against that at any time you want (read: 90% of billionaires), then you have that money effectively liquid and untaxed.

Then they will take out a bigger loan to pay off the existing loan and as such, pay almost nothing in taxes, the stocks that they borrow against grow faster than the interest on their loans, and they can repeat this process until they die where their debt just gets eaten by taxpayers because they transfer assets at the right time to children and then their estate will pay back the debt after death (just transfer stock ownership to lenders probably)

Even if the stock market crashes during this time, they can declare bankruptcy, free themselves of their debt, and then sell assets and they already have a huge golden elevator ready to bring them back up.