The problem is that when they purchased the iPhone, it came with said cable.
I purchased an iPhoneX this year and it comes with, you guessed it, a USB-A cable. So in order to connect it to my new Apple laptop I need to buy either a $20 cable or a special dongle? If Apple wants to go all in on USB-C then they shouldn’t still be shipping their top of the line “futuristic” phone with USB-A cables.
But obviously they already know that they would have gotten many dissatisfied customers complaining about having to buy a cable just to ‘connect their phone to their iTunes’ if they did that.