they could earn by asking users to deposti bitcoin on their bank, and instead of playing with fiat they play with bitcoin(invest them) they in exchange will offer you interest on your deposit(like 2-3%) and security
Interests are paid on fiat deposits, because fiat is susceptible to inflation. It loses value every day. Inflation is not a problem with Bitcoin, as the monetary supply is limited. So there is no need to pay interests on BTC deposits.