You've seen them and not given them a second thought, they're so ubiquitous. But think: Buttons and pressure plates, commonly used for traps or alarms in medieval settings, don't do much of anything without electricity. If you mechanically connected them to what they're supposed to operate, you'd either have to press them in several feet or push with the force of a truck! And don't even get me started on fires that start or spikes that come out of the ground when you get near them. Sometimes there are even doors that slide up into the ceiling at the press of a button, and slide back down when it's pressed again. True, a mechanism to do that is possible. But it'd likely be larger than the building it's used in! Admittedly, counterweights would work for the levers and buttons at least, but they'd be difficult to maintain, to say the least. And certainly not work after centuries or millenia of abandonment. Plus, occasionally a switch somehow manages to mechanically transmit a force across half a continent! There are so many failure points in a system that long it'd probably break before they were done building it, and that's not an exaggeration.
Hello, Unknown Troper. You'll need to get known to lend a hand here.