IBM Mainframes are designed to be serviced while running so if you have multiple CPUs you can offline one at a time for upgrade it without the whole mainframe going down. Big Sun Solaris boxes where built like at as well.

If your mainframe had only one CPU, you did have to turn it off in order to service it. But you could upgrade the OS without turning it off. While they aren't cool tech now, mainframes are a marvel of hardware engineering.

plus, i would imagine turning them on and bringing them online isn't just a press of a button.