Your comment got me thinking, and I checked the math. It turns out that light takes ~0.2 ns to travel 2 inches. But the speed of signal propagation in copper is ~0.6 c, so that takes it up to 0.3 ns. So, still pretty small compared to the overall latencies (~13-18 ns for DDR5) but it's not negligible.

I do wonder if there are nonlinearities that come in to play when it comes to these bottlenecks. Yes, by moving the RAM closer it's only reducing the latency by 0.2 ns. But, it's also taking 1/3rd of the time that it used to, and maybe they can use that extra time to do 2 or 3 transactions instead. Latency and bandwidth are inversely related, after all!