USB-C actually has pins dedicated to application/debug use (SBU1 and SBU2).
"SBU1 and SBU2: these are low-speed lines used only for Alternate Mode and accessory mode. For example, with
DisplayPort, AUX+ and AUX– transmit over the SBU lines. For audio adapter accessory mode, these lines are used
for the microphone input and analog GND."
Cost. It's a clever hardware hack that saves money, and 1 cent saved over 100 million units is $1,000,000!
Mining rigs were using USB-C to transport PCIe x1 by the truckload. Because USB 3.0 cables are dirt cheap and are capable of transporting signals at PCIe x1 data rates.