Lenders have an amount of capital that they need to invest and earn returns -- they're generally not in the business temporarily so they don't want their capital back. And when the loans are secured by hard assets, e.g. publicly traded stocks, there's little risk of default so long as the price stays up. In times of rising stock prices, there's little to no reason for a debt holder (lender) to exit their positions at maturity. Rather roll and continue taking the return (interest).