Until last year, all iPhones had Lightning ports, and Lightning cables only support USB 2 speeds anyway. Possibly they could have made special Lightning cables or adapters in order to enable USB 3 over Lightning, but I guess even Apple could see that would have been a bit ridiculous.
Pretty much all other Apple devices (eg: iPads) that actually have USB-C ports do support USB 3 speeds. The non-Pro iPhone 15 is an anomaly here.
That’s how Apple is upselling iPhone 15 Pro, by making artificial limitations on lower tire devices.
Apple is always playing this game and it will continue to be this way.