well, probably because in upper-level dressage they are mostly showing off the RIDER's skill, and one thing a treed saddle does well is really assist the rider in maintaining a solid position. Especially on one of those big wide bouncy warmbloods- a nice narrow twist-saddle with lots of structure to help you sit that powerful trot is more important than a healthier horse back.
For example, one question for which I have not yet heard an answer is why, if treeless saddles promote a better, healthier back, do you not see them in upper level dressage competitions? These are riders that do everything possible to showcase their horse's gaits. What makes a saddle suitable for endurance (where they are widely use) but not dressage?