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).
I don't know how these specific loans are structured but in real estate it's relatively common for a loan to be interest only with a balloon payment (the principal) due some number of years in the future. So in theory you could just pay off the balloon payment with a new loan and repeat the process.
Lines of credit against assets are typically interest only, interest rate pegged to however many basis points above the current fed fund rate.
There is no balloon payment ever due if you simply pay off the interest indefinitely.
Of course there is always the possibility of a margin call against the loan where if you lose X% of value on the securing asset you may be liquidated of it and the proceeds used to pay off said line of credit.
There are a million caveats and different loan structures so I’m sure some finance bro will be along to correct me shortly. But overall for normalish folks this is more or less the correct mental model.