There are already slats so the only hole you can get a fork into is the earth, unless you’ve already got something convincingly shaped like an earth pin in the earth hole to open the slats over the live and neutral. If you’re going to that much effort to zap yourself, the switch isn’t going to be much of a hurdle.

I’d suspect that it’s largely because it’s more convenient to have a switch than to unplug things and plug them back in again, especially as our plugs are a nightmare to step on to the point that Americans complaining about stepping on lego seems comical to anyone who’s stepped on lego and a plug.