some issues (pulled out of thin air):
- existing power grid and batteries for UPS would maybe be too weak.
- heat management is already non-trivial and they planned their data centers for their target load. adding GPUs would also mean adding new cooling system.
- cpu and network load is already a non-trivial issue for their map/reduce algos. spilling p2p traffic on top of that would not really help.
- if they invested in hardware it would not be GPU but FPGA or even ASIC.