Yeah, I think it's hard to keep your computers at 100% utilization for the entire day. You host services close to your users, and your users go to bed at some point, many of them at around the same time every day. Then your computers have very little work to do.

Some bigger companies have a lot of batch jobs that can run overnight and steal idle cycles, but you have to be gigantic before that's realistic. (My experience with writing gigantic batch jobs is that I just requisitioned the compute at "production quality" so I could work on them during the day, rather than waiting for them to run overnight. Not sure what other people did, and therefore not really sure how much runs overnight at big companies.)

Cloud providers have spot instances that could take up some of this slack, but I bet there is plenty of idle capacity precisely because the cost can't go to $0 because of electricity use. Or I could be completely wrong about workloads, maybe everyone has their web servers and CI systems running at 100% CPU all night. I've never seen it, though.