Seems more like electricity pricing, which has peak and offpeak pricing for most business customers.

To handle peak daily load you need capacity that goes unused in offpeak hours.

Why do you think that this means "idle GPU" rather than a company recognizing a growing need and allocating resources toward it?

It's cheaper because it's a different market with different needs which can be served by systems optimizing for throughput instead latency. Feels like you're looking for something that's not there.