The US actually has two real-time payment systems: RTP and FedNow. The number of participating banks is growing rapidly.

https://real-timepayments.com/Banks-Real-Time-Payments.html

Prior to that, you could get instant transfers but it came with a small fee because they were routed through credit card networks. The credit card networks took a fee but credit card transactions also have different guarantees and reversibility (e.g. costing the bank more in cases of fraud)