I still don't get why iPhones don't use USB-C. OOTB, there's issues between a brand new iPhone and a brand new MacBook Pro.
The old 30-pin connector (inherited from the iPod) had various issues so I think Apple was eager to replace it. The lightning connector was their solution. It predates USB-C by a few years, so that wasn't an option at the time (I guess it might have been on Apple's radar by the time the lightning cable was introduced, but if so, they must have made the call not to wait.)
Since USB-C has made its way to some iPads, my guess is Apple is in the process of phasing out lightning connectors entirely.
It could also be more that Apple is trying to position the iPad Pro as a "laptop replacement" - and their laptop has type-C ports - so having the same port makes it feel and work more like a laptop.
And it’s possible both viewpoints have merit depending on what aspects of the standard are considered significant.
More generally, the lightning port is actually slightly slimmer than a USB-C port, which is important. While the iPad isn't any thicker than an iPhone, it has squared-off edges, versus the iPhone's rounded edges, so switching the iPhone to USB-C would likely require either making it thicker or making the area around the charge port flat.