OP, don't worry about this becoming a "debate". That's not a bad thing. This is a subject we at HF have rolled around , over and over again. It's very interesting to hear each person's thought, their learning from books and their personal experiences. I appreciate the Bsms wants to ask the question "why" when things long accepted in "classical" teaching are raised as correct.
We probably should ask why. We can assume, with some degree of security, that something that has been taught for centuries as the correct position has a long trial period to it, so likely to be correct. But, still, one can ask, "why?"
An excellent question.
I do have to disagree with bsms's comment:
A dressage seat is also well behind the motion, unless the horse is moving in a collected gait - and since that is the goal of dressage, that is what the dressage position is intended for.
I don't know why he would think that. Behind the motion? H m m ,.... not if the rider is riding correctly, be it walk, trot or canter. If the saddle is designed and balanced correctly to the hrose, and the rider sits up correctly, they should never be behind the motion, whether it is collected or extended.