Being in debt doesn't make you poor. Loads of millionaires/billionaires are in debt. There are reasons why a millionaire/billionaire would choose debt over just paying outright.

If the interest/capital gains of their assets is higher than loan interest rates, then paying outright would be a net loss.

Invested assets: 10% annual gains Loan interet rate: 5% APR

In the above case, keeping the assets invested and taking on the debt would still net gain the remaining 5%.