PC desktops use DIMMs for RAM. They can stick whatever they can find in there and if need be adjust the voltage and timing to make it work.
Apple needs the One True Memory Chip. They are not as flexible. Same issue for the SSD. Commodity NVME or perfectly specced chips.
Apple goes for peak performance per watt, The PC desktop is for flexibility and low cost and damn the power consumption. A top of the line NVME pulls about the same power as the base M4 SOC. Since the GPU is pulling down 500-600 watts the NVME doesn't really matter as long as you remembered put the heat sink on it.
At the high end of gaming PCs the supply is also suffering. Jay-two-cents on YouTube was complaining about that.