USB, especially USB C, is very much designed to carry power. Not quite as much as high end graphics cards guzzle these days but it goes up to 240W. Ethernet, HDMI, DP and even VGA (with extensiosn) are also all used to carry power even if much smaller currents.
It's designed for 5 amps. In this context, that's close enough to "not carrying power".
If we're considering the bigger voltages that allow higher power on USB C, then the existing GPU plugs are fine because we can deliver 600W using the same current as a 150W 8 pin plug.