Yes, there are specialized products catered to billionaires. But those aren't getting them better rates than someone with a $200k portfolio (Zuck is not conventionally a less risky borrower than the Options Clearing Corporation!). They exist to work around the fact that some borrowers can't just casually liquidate their stock on the open market, let alone at face value. By all accounts these products are more expensive than retail.
Mostly this is an expensive (but maybe still less expensive than taxes, depending on the rate environment—it's more of a no-brainer in ZIRPland) way to diversify out of a single-stock portfolio without selling by adding leverage. At Zuck's age, it's still very unlikely to make sense to borrow instead of sell to spend. He's been known to pay real taxes in the past, they just look small relative to his imputed wealth growth because rich people don't spend a lot relative to their wealth growth because they, quite by definition, have a lot of wealth.
Any asset a bank is willing to take is collateral has the same issue, it’s just very pronounced in this instance.
If you take your idea at face value, anyone who borrows against their property to renovate/upgrade would be up for tax.
Also, you'd totally gut retail home equity lending as collateral damage, with disastrous social policy consequences.
It's similar to a reverse mortgage. Say Fred and Wilma own a house worth $4MM with no mortgage on it. With a reverse mortgage a bank will lend them $2MM. Fred and Wilma make no payments and continue to live in their house, spending the $2MM while the interest on that loan just increases the amount they owe the bank. After both Fred and Wilma have passed away the house is sold and the proceeds are used to pay back the outstanding loan. If there's still money left over, it goes to their heirs. If the sale comes up short, the bank loses money, which is why these reverse mortgages are typically less than 50% of the value of the house and they typically have higher interest rates than conventional mortgages. From Fred and Wilma's point of view, they can use the value of their house now, while continuing to live in it. They essentially spend their children's inheritance.