Ah, but they do. I've been going through my collection and sadly 2 of my 4 PowerMac 6100's were destroyed. Motherboard, power supply, and case on both of them. I was able to salvage the floppy and hard drives.
I'm pulling batteries from all of my computers in storage because of this. One of my Mac II's still has soldered batteries that are good (amazing), so that computer will be permanently plugged in and checked regularly.
I am also going to set up a regular schedule to power them up for the sake of the capacitors.