The reason the rich borrow money is to take advantage of tax loopholes. It's not about being reasonable or what ought to make sense. They are gaming the system, that's it. So, how does it work?

If they have investments in the stock market, then they get taxed when they sell those. So even though the investments are usually going up in value, they don't want to sell too often. But they still need to buy things.

So, where do they get money for living, houses, cars, travel, etc? If they get paid for working a job, their income is taxed a lot, meh. If they sell their stocks, they get taxed a little, meh. But if they get a low-interest loan, that money is not taxed.

And you might say hey, money's gotta be paid back some day. But remember, the goal is to find the loopholes, the places and times where either you don't pay tax or you pay much less tax. And those loopholes are all over the place. In the end, the details are just boring. Most financial scams have just enough moving parts to look amazing, but if you take an hour to figure them out, it's nothing exciting.