I suspect it is because that's the way it has always been done, until one day, someone said "hang on a minute.......why not park them vertically and save drag, noise and also the torque effect of the drag on the motor and pivot, thereby allowing a smaller and lighter motor/gearbox?"
If aircraft manufacturer(s) were seeking to reduce as much drag and weight as possible in the pursuit of greater efficiency, then even horizontally parked wipers would come under scrutiny.