Look at PCIe connectors pinout, it is very interesting thingy.

As its origins can tell you a lot about committee, bureaucrats who are present in them and highly specialized engineers who are forced to go around stupid decisions.

Bureaucrats reversed wires assignment compared to ATX CPU connector, in absolutely stupid attempt to provide reason why they must be paid for their work.

During design bureaucrats realized that many capitalist manufacturers will put crap thin wires in their PSU for PCIE power delivery. Proper sane man will work with ATX standard and set strict specifications to always use 3 wires for 12V 6 pin connector (and 4 for 8 pin one) and 3 wires for ground, copper ones and no smaller than specified gauge. But no.

Decision looked smart for bureaucrats but in reality it is utter stupid. They added special sense wire so properly designed PSU will sense actual voltage drop across the crappy wires and adjust +12V going only to this connector to compensate for this drop.

Well, at first. It is cheap crappy power supplies that save each cent and use thin alu wires. They won't do extremely advanced implementation you want from them. And expensive power suppliers with good thick wires do not need it much.

Second. It requires separate step up DC-DC regulator for each PCIE connector inside PSU. It is around 20x more expensive than to use good wires. I am not even talking that none of standard IC for PSUs have support for such features or that it is not much space inside PSU for all this shit.

Third. GPU manufacturers are also not stupid and know that they can't rely on top quality +12V values at mass market cheap PSU, so they try to design their power stages with good margin or voltage drop.

Perfect example of people who got lot of money, wasted lot of time and instead of simple copy of ATX CPU connector induced problems no one asked for.

It is all something like this one Youtube sketch, just in real task: