Laws of thermodynamics
You can treat basically all electrical devices as close to 100% efficient heaters. All the energy has to go somewhere. Whether it's a gpu or cpus it's all ends up as heat in the end. A tiny amount will leave in the network cables, but still ends up as heat in the next device.
AC works by transferring the heat to a medium, pumping it elsewhere and releasing it. Convientiently it can move more units of heat than it consumed.
I had always just assumed that ac dynamically scaled with the workload. There have been temp probes in every dc I hadn't been in. Why was thier not done from the start?