Having separated GPU memory also has its benefits. Once the data makes it through the PCIe bus, graphics memory typically has much higher bandwidth which also doesn’t need to split with the CPU.
The benefit when having an ecosystem of discrete GPUs is that CPUs can get away with having low bandwidth memory. This is great if you want motherboards with socketed CPU and socketed RAM which are compatible with the whole range of product segments.
CPUs don't really care about memory bandwidth until you get to extreme core counts (Threadripper/Xeon territory). Mainstream desktop and laptop CPUs are fine with just two channels of reasonably fast memory.
This would bottleneck an iGP, but those are always weak anyway. The PC market has told users who need more to get a discrete GPU and to pay the extra costs involved with high bandwidth soldered memory only if they need it.
The calculation Apple has made is different. You'll get exactly what you need as a complete package. You get CPU, GPU, and the bandwidth you need to feed both as a single integrated SoC all the way to the high end. Modularity is something PC users love but doing away with it does have advantages for integration.
An M2 Ultra has 800GB/s of memory bandwidth, an Nvidia 4090 has 1008GB/s. Apple have chosen to use relatively little system memory at unusually high bandwidth.
