That would be more efficient, but GPUs were banned to level the playing field between extremely rich and slightly less extremely rich teams.
Of course, the people who made that rule probably just looked at the cost of a GPU vs the cost of a CPU without realizing that per operation GPUs are much cheaper to run.
EDIT: I originally heard this on Reddit and after looking I can't find any real sources, so this could be inaccurate.
You should have a quick read through the document, that's just one rule.
The rules seem mostly focused on computations rather than hardware.
I'm not that familiar with the hardware-level stuff they're talking about, but it seems likely that there are situations where a CPU is preferable to a GPU under these rules.
Of course, the people who made that rule probably just looked at the cost of a GPU vs the cost of a CPU without realizing that per operation GPUs are much cheaper to run.
EDIT: I originally heard this on Reddit and after looking I can't find any real sources, so this could be inaccurate.