But up/down gives you an easy to remember key, up os ON, down is OFF. Sideways, you would have to use right/left, not as easy to remember.
edit: in fact I have 2 light switches in my kitchen sideways, because I wanted 2 switches in the space normally occupied by 1 switch.
Sideways flip horizontal toggle switch plates were commonly installed in homes in the 1970's and 1980's and are still available today.
So you don't turn them on/off accidently when you walk by.
You are carrying a Priceless Ming Vase into a room at night and accidently brush the light switch. the lights go off...oops.
Mounting the boxes is easier, national preference, less costly but mainly ease!
It makes the most sense and has been done this way for decades.
