Why are some rich people always borrowing money?
Because by borrowing money you can leverage your investments and make even more money.
Let me explain by an example:
Let say you invest 500 000 euro/dollar whatever into a house and the return every year on your investment capital is 10% = 50 000.
When you can borrow money and only pay a part of the sum your return will be a lot higher. For example you pay half and the bank pays the other half. Then you pay 250 000 and your return is the same 50 000 so percentage wise that's a 20% return. You also have to take into account the money you have to pay back since borrowing is not free. But as long as your return on the capital you borrowed is higher than the fees for the bank, then you make a profit. For example when a bank charges you 4% interest to borrow money, and you can earn 10% or more on that money then it makes sense.
It's not that rich people borrow money because they don't have any themselves. They do it to maximise their returns.
I think it works especially well with real estate: https://amzn.to/2E5Ewgn