A growing number of financial start-ups are being introduced, each hoping to usurp the giants of Wall Street by offering better services and lower fees. They see banking as a prime candidate for disruption because it tends to run on bloated, dated technology. Even applying for a loan can take weeks or months, an eternity in the on-demand world of tech innovation. But tech entrepreneurs have found starting a bank to be an arduous ordeal, made more ­difficult by a maze of regulatory requirements and drawn-out compliance processes. It also requires a tremendous amount of money.  These realities have forced many entrepreneurs who hoped to replace banks, such as Simple, to scale back their ambitions and partner with existing banks.