The US banking system is notably decentralized/fragmented, with a fat tail of regional banks. That extends to regulation, where banks are chartered at both the federal and state levels (both of which jealously guard their authority). Large banks started to offer a direct-transfer service called Zelle[1] a few years ago, but making it "standard" is a slog thanks to all of those regional banks. Instead, most people use third-party apps that operate over ACH.