And more importantly, M2's energy/bit is pretty low.

* Shared bandwidth between CPU and GPU - as soon as you use up one, you're bottlenecking the other.

* No M2 memory benchmark I've seen gets above ~250GB/s once cache effects are exceeded.

* Random access latency is >~110ns which is no improvement over even mid-range DDR4 designs from 5 years ago.

TL;DR: It's about cost