I get that. BUT, the risk is higher. They are taking the risk that a loan at 2% will yield returns at 8%, giving themselves a 6% profit. It is the same reason why we borrow for houses and banks lend. We believe that even though I will pay double for my house over 30 years, after 30 years my house will be worth more than double what I paid for it.
I'll give you an example. RC Willey was owned by two brothers. When they had TWO stores, they decided that they would get debt free. They did that, and wondered what to do with their cash. They decided that they would loan their customers the cash they had, with 18% interest.
A few years ago, Warren Buffett bough RC Willey 175 million. Was it because he had interest in the furniture side of things? NOPE. It was because RC Willey was the fourth largest bank in Utah. They got out of debt in 1978. Their net worth was 2 furniture store business. What would that be? Let's aim high and say a million a store. So, in twenty years, their net worth went from 2 million (which is probably ridiculously high) to 175 million. Your billionaire friends aren't getting returns off their debt like that.
I get debt. It is useful, and risky. It is nowhere as good as cash.
Your example also had risk. The Willey's purchased the furniture with cash and then loaned it out .. same risk as borrowing money to purchase the furniture and then selling it for cash, sans the interest rate.
I did the same thing. A company I had sold in '04 was purchased less for the business, less for the physical assets, and much more because I had millions in performing consumer loans averaging 16% interest.