My guess is they source fiat from people who don't believe that and volatility. It makes no sense to keep a loan open when the borrower can buy more than what they put down as collateral with the borrowed fiat. I'm sure these borrowers also wouldn't want their Bitcoin immediately sold either. It would defeat the point of borrowing the fiat for them. The simplest solution (from what I can see, I don't use the products) is to build in extra Bitcoin to help avoid immediate liquidation.