I have a picture of me standing on my boy....I also stood up during a stop in a parade once to wow some little kids...it was a small local parade and im basically the only rider in it...the kids love seeing me do weird little things like that...but its more of me being trained then the horse...he just stands there. Im the one who has to work hard for balance lol
It just isnt a selling point...any horse that will stand still long enough can usually be stood upon....