It is because GPU manufacturers do not know and cannot control what power supply unit (PSU) you are using.

So, they have to include an adapter for the worse possible scenario that people may use a complete garbage level or substandard PSU with their GPUs, and the GPU should still be able to draw sufficient power via the 12VHPWR port.

But unlike Nvidia and AIBs, you know exactly what you are using! This is the difference.

If you are using a PSU from a reputable brand (e.g., Corsair, Seasonic, EVGA, Asus, Cooler Master, etc.), then you are perfectly fine to use 2 x 8-Pin or 3 x 8-Pin to power your 4090 with full 600w mode.

Note that original 12VHPWR cables from these PSU manufacturers are all using 2 x 8-Pin too.
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