It's not, because if you plugged a device needing 600W using an MPX connector, it wouldn't work at all, since MPX can only do 500W.
Here's the thing. When you have a lot of amps, they create noise in wires next to them. They'll also need thick wires and will get hot. When you have a lot of volts, they'll go through shielding and start a fire.
Do you really want to place something with a lot of amps and volts next to something that's transmitting timed high-bandwidth data, like a graphics card? Or run the entire output of your big PSU box that needs a bunch of fans, through the motherboard? For a tower form factor?
The single slot for power and data is convenient for phones and laptops. Not something that's 600W and is going to power a space heater in its next kilowatt+ iteration.
the RF noise form computer components is enough to be taken care of by regular metallic shielding, because the amps are low for actual compute operations. an inch of space on its own is an effective barrier. this is very different than having a wire running enough amps to power a space heater, located next to your data transmission lines. even having the GPU in its own external enclosure, you would not want the data cable to be bundled with the power cable for anything that pulls over half a kilowatt.
Here's the thing. When you have a lot of amps, they create noise in wires next to them. They'll also need thick wires and will get hot. When you have a lot of volts, they'll go through shielding and start a fire.
Do you really want to place something with a lot of amps and volts next to something that's transmitting timed high-bandwidth data, like a graphics card? Or run the entire output of your big PSU box that needs a bunch of fans, through the motherboard? For a tower form factor?
The single slot for power and data is convenient for phones and laptops. Not something that's 600W and is going to power a space heater in its next kilowatt+ iteration.