because their current lighting cable only transfers at USB 2.0 speeds as well

This is the main reason. People are confusing the protocol (USB 2.0/3.0) with the connector (USB C/Lightning). Apple slapping a different connector on the phone isn’t changing the underlying technology inside the phone. People claiming that USB-C must mean 3.0 are just spreading FUD in order to shit on Apple again.

And, as others in this thread have pointed out, high speed transfers by cable are low priority for phone users, there are much better tools to do that, like maybe an external hard drive.