Earlier quoted context omitted.
On the other hand, the 6th gen Mini uses an A15, and has USB 3 speeds support (only 5Gb but still). So apple might have had USB3 speeds support in the A15 specifically for the mini, or they might have included USB3 support for a while in all SoC and enable it selectively for segmentation purposes. That would gel with the comparison between the iPad Air 5th gen and the iPad Pro 5th gen: they both use the desktop-class…
The 4th gen iPad Air uses an A14 and also has USB 3 support. So Apple is using a PCIe USB 3.0 controller for USB 3.0 iPads. They could've easily done that on the iPhone 15, so it's market segmentation.
Adding additional chips/additional controllers is not "easily" done. The iPads have more space to spare on their boards, and have larger batteries that can absorb the additional power requirements for running those additional chips much easier.