iPhones don't have USB 3+ because the Lightning connector only has one differential pair for data. If iPhones had a connector with more pairs (USB-C), I doubt it would cost much of anything to add a USB 4 block to the SoC (which they already have on many Apple Silicon SoCs) and wire it up to the connector. Sure, there's more to it, but it's not rocket science.

Presumably, the thinking goes that vendors will use the opportunity to have a large bandwidth.