Why 12v though? What requires that much voltage? The chips are running at a few volts. Spinning drives are gone, is it just for fans? Seems silly to provide 12v then shift it down again.
True, but motherboards need to shift down anyway. The motherboard controls the voltage of the CPU, and that can be anything.
While I don't know the exact component that needs 12V, you have to keep in mind that your typical computer draws about 300W under a small load. 300W/12V means 25 amps. Reduce the voltage without lowering the power consumption and you need bigger wires to handle the increase in amps.