128GB of the slowest possible DDR4 in 8 DIMMs is still faster than 16GB of the fastest DDR5 in one DIMM. 12.8GB/s*8 = 102.4GB/s > 74GB/s
It requires many more traces, but you still get more throughput overall with more, cheaper memory further away.
The packaging costs on the GPU do go up for the extra traces.
More critically, no one would design it like that. There's a memory hierarchy. You would likely be a small amount of high-speed soldered-on RAM next to the GPU, and more slightly further out.
Until you do the math.
128GB of the slowest possible DDR4 in 8 DIMMs is still faster than 16GB of the fastest DDR5 in one DIMM. 12.8GB/s*8 = 102.4GB/s > 74GB/s
It requires many more traces, but you still get more throughput overall with more, cheaper memory further away.
The packaging costs on the GPU do go up for the extra traces.
More critically, no one would design it like that. There's a memory hierarchy. You would likely be a small amount of high-speed soldered-on RAM next to the GPU, and more slightly further out.