The issue is that power savings stuff is often bad on Linux... especially on older hardware. 'Idle' in this context means running with near zero load since a server cannot suspend or hibernate. In theory you could setup hibernate/suspend + WoL but that tends to not work well.

Some power saving features will really only save 5-10W and can make PCIe cards unstable, poor hard drive performance, laggy keyboard & mouse, etc.

In particuler, GPUs often have poor suport for the normal power saving features. I just pulled the dedicated GPU (GTX 760) out of my old machine that is now a headless server and that saved ~20W. What's weird and bad is that that 20W was being consumed without a graphical interface even running.