Servers sitting idle is a strange concept. Ideally those resources should be powered down and workloads should be consolidated until the machines reach an optimal level of utilization.
Global utilisation rates are very low. 5~15%
I guess the reality is that solving that would require a practically impossible level of coordination across the industry.
https://sardinasystemsblog.medium.com/how-can-an-enterprise-...
That's not to say you shouldn't make attempts to conserve power with better performance scheduling on the CPU.
You can’t boot a full OS in seconds, but you can boot a thin hypervisor and have compute resources immediately available. Same applies for hard disk drivers that can be spun down or flash devices that can be unpowered when not in need.