The pcb footprint isn't the same. Changing the port isn't as simple as just soldering this one instead of that one. The pins are in a different order. Changing the port requires redesigning the PCB that the port attaches to. In a larger device, maybe you could create some kind of internal adapter that can solder to the lightning footprint but provide a usb-c port, but there's just not enough room in a modern phone.

Plus, even though the USB portion of the circuitry may be the same, the port does more than just provide a usb interface. There's also headphone functionality and other things on there that have to be adapted too, or the phone will have less functionality than it did and people with bitch that apple broke their headphones when they changed the port over.