Because it's not USB 3.0 for starters. When I received my brand new ultra powerful iPhone 13 Pro Max and had to transfer 13,500 songs (all 320 CBR MP3) to it via WiFi or USB 2.0 you can understand it took hours to complete. In-fact I think it took around 8 hours and I'm not exaggerating.
USB-C has lots of uses, not just that I could carry only 1 power brick and charging able with me (MacBook Pro, iPad Pro and iPhone could use the same brick and cable) but I'd get the insanely high transfer speeds when I need it.
I also shoot a lot of video with my iPhone and I'm looking forward to the Pro Res format, I use Final Cut as my editor of choice. You know how long it's going to take to transfer the 50GB to 100GB 4K files off my iPhone to my Mac over USB 2.0 haha I might as-well start it before I go to bed and hope it's done in the morning.
Again the transfer speed is 480Mb/s (33MB/s) vs 10Gb-40Gb (1,250MB/s-5,000MB/s) on USB-C depending on the protocol. The storage in the iPhone is actually very fast, well capable of over 200MB/s as Apple uses good NAND flash a good controller and NVMe as the storage protocol, we could get 10-15x more performance for file transfers if we had USB 3.0 speeds.
The fact is sadly the lightning connector doesn't have enough usable pins for those kinds of speeds like USB-C does. Just like the iPod connector before it, it must go.