I’m not sure what you’re trying to suggest here… the PSU is also connected to the wall with a C13 connector, and is able to supply 600W at 12VDC to the card.
Yes, but the voltage on the wall side of the psu is not 12V it's 120-240V. And 600W at 120V requires only 5A of current.
It's easier to handle more power at higher voltages because heat produced depends on current and not on voltage.
If you take that puny C13 connector and move it to the 12V side with its 50A it would just melt.
