That's not a dumb question. As alluded to below, for most people the main challenge is their software stack. Beyond that, 200k+ vcpus is still a ton so you'd need hundreds to thousands of GPUs to just match the flops (Note: I don't know if Drew's code is vectorised or not on CPUs).

Put together that by using Preemptible VMs (and yes, apologies we still don't offer GPUs as preemptible) it's economically rational to use spare CPUs.

Disclosure: I work on Google Cloud.