It doesn't take an EE to look at 2 facts:
1) A 15pin SATA power connector is powered by 5 wires: 1x 3.3V, 1x 5V,
1x 12V, and 2x ground.
2) A typical PCIe 6 pin connector is made up of 6 wires:
3x 12v, and 3x ground.
It's a very similar situation as to why you only buy a Molex->PCIe adapter with two plugs - you never want all the current (75W @ 12V = more than 6A) running thru a single wire.
Also, most SATA cables have more than one connection, and they often share a 12V wire. Could you imagine 3 or 4 PCIe 6 pin adapters, all trying to pull 75W each thru a single 18awg wire? That's almost 25A!
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