It's irrelevant if you can't balance the cost, CPU, shared DRAM, and - possibly most importantly - PCIe and network connectivity in and out of it. A typical mid-range Intel Xeon has 40 PCIe lanes. A high-end GPU requires 16. You can do the math. Facebook has decided upon a balance that works for them and their workloads; it's quite likely that going to 16 GPUs in the chassis resulted in overall worse utilization because of PCIe bandwidth limits, or socket and QPI count, etc. What's a big deal is someone having gone and done a lot of work to calculate and experiment with finding a balanced design that's also cost-effective and easy to maintain.