In Turkey where the wet room is the norm then can see why switches outside, oddly the Turkish style wet room under UK rules could have a socket in the room, as there is no bath or shower in the room, just taps and loose bowls.
But in the UK no real reason for switch outside the room, however dads house 1954 a block of 4 switches did living room, best room, hall and stairs + landing all in the hall, not a clue why they were not put in the rooms, and we would regularly in error flick the wrong switch, so living room lights switched on without realising when going to bed.
I will guess some historical reason for having switches outside the room, but the wall gas lights were at the light its self, and ceiling gas lights often pull chain. So no idea why switches were not in the room.
It was normal to have pull cords in bathroom and toilet so wet hands would not matter, but if you have wet hands it does not matter if switch outside or inside the room you still have wet hands, so still needs pull cords or switches designed for wet conditions.
So see no real reason for switches outside the room, what ever the room is.