Because then you'd have to change the interest rate charged to loan holders to a million percent, and those people would close their loans rather than pay that interest rate.
But why are the lenders even paying 6% when they could go get a SoFi loan for four percent and on top of that have the balance in a more convenient form?
Because SoFi doesn't allow you to use Ether as collateral for a loan. Many holders of Ether may not necessarily have the credit required to get a loan on more favorable terms.
