The 150W "spec" comes from PCI-SIG, driven by Nvidia, as well. The had already defined that the reason for the 6-pin on a GPU was because the GPU needed 75 more watts of power than what the slot could deliver. Needs 150W? Put in a second 6-pin. But when cards started needing 225W additional, Nvidia thought it would be silly to put a third 6-pin on the card. So they asked PCI-SIG if they could put two sense pins on an 8-pin connector so, if grounded, the card would "know" that it was safe to demand 150W from a single connector.
Think about it logically. Surely you would expect 6 conductors on a mini-fit jr. connector to deliver more than the equal amount of power as 4 tiny pins in a PCIe slot. And you don't magically double the capacity of a 6 conductor connector by simply adding two sense pins. If you want to double the the capacity, you MORE THAN double the conductors. But that wasn't the goal here. The goal here was to specify the power demands of the card: 75W, 150W, 225W, 300W, so on.
The cheaper terminals, of reputable brands, use the brass w/ tin terminals with 18g wire. In a 2x4 configuration with 2x3 terminations, those terminals support 8A per conductor. So, your "typical" 6-pin PCIe, or even 8-pin PCIe (since it's still technically only 6 power conductors) is capable of 24A. So, at 11.4V (because we always work +/-5%), you're talking about 273.6W per connector. Fine for a 6-pin and 8-pin on the same cable, but not good for two 8-pins which is when they tell you not to daisy chain. It's also why Nvidia's squid adapter has three 8-pins and not just two.
Some manufacturers use mini-fit HCS terminals. These are rated at 10A per terminal in a 2x3 configuration. So using the same math, you have 342W per connector assuming voltages drop to 11.4V.
The cable Corsair made here:
https://www.corsair.com/us/en/Categories/Products/Accessories-|-Parts/PC-Components/Power-Supplies/600W-PCIe-5-0-12VHPWR-Type-4-PSU-Power-Cable/p/CP-8920284 uses 6x mini-fit HCS terminals per 8-pin connector. So two of those type 4 connectors are capable of a total of 60A, which is, obviously, GREATER than the capability of the 12VHPWR connector.