At least in my experience, for highly optimized code (which I imagine their's is), above certain core densities you run out of memory bandwidth, PCIe lanes, etc before you run out of cores unless your workload is particularly compute intensive. Power consumption also needs to be modeled into the hardware cost, so higher TDP does matter. It is a cost optimization problem, and those 64-core parts may offer little marginal gains for the added cost if bottlenecks are elsewhere in the system.

My experience buying CPUs for data intensive servers has typically been that maximizing the performance-density-cost curve often recommends a mid-range number of cores at a lower-middle clock rate. These CPUs are inexpensive relative to their product families while still having enough horses to drive your memory, PCIe, etc to saturation with highly optimized code. Just enough resources, but no more.