Imagine a voltage of 5 V, to draw 100 Watts from that you need a current of 20 Amperes. The german DIN 57100 requires a cable of 4mm to provides such a current.
Voltages inside a PC are usally 5V and 12 V. For 12 V you still need sort of 8,333 Amperes, that means 1mm for the cables. The other problem will be the plugs and sockets. 20 A is enough to sort of solder the contacts together or start a small fire if there is too little contact between them. Ok, lets imagine a device that can negotiate 100 Volts, then you just need 1 Amperes for 100 Watts. Fine, but that means that the USB connection now needs to be treated like a healt hazard because its over 65 Volts and can proivde more then 50 mA.
I'm really curious to see how they will jump over that physical obstacle.