As long as they satisfy their capital requirements, banks can lend as they please. They tend not to make really bad loans because directors are legally beholden to shareholders to ensure management is appropriately managing risk. The Fed doesn't tend to print trillions of dollars for no particular reason because it is legally beholden to its dual mandate. It's not that different. Yes the Fed can create "reserve currency" and banks can only create "commercial currency" but the distinction is a technical one and doesn't have a whole lot of macroeconomic impact.