The PSU also powers the motherboard (keep in mind all those power hungry USB-C PD peripherals), CPU, cooling (fans and AIO liquid cooling pumps), storage, and RAM. Furthermore, when building a PC one would keep some buffer of a few hundred watts in the event that one would want to down-the-road add another GPU (or two (particularly for ML work)) to spare PCIe slots of the motherboard, or simply future-proof the PC for future more-power-hungry GPUs.
