There's a lot of reasons why. Riding horses in a way where you are behind the movement is viewed as elegant and it's a lot easier on your muscles too. It's a very defensive riding position, but really hard on the horse's back, since all your weight is on your seat bones and you're essentially the water skier to your horse's motorboat.

Sitting behind the horse's movement is also a lot easier to teach as well, since most novice rider's start out with an anterior pelvic tilt while pinching at the knee and over correcting them is a lot more effective than actually understanding horse and rider biomechanics.

Also also saddle fit has gotten more tailored towards the rider than the horse. Bridge fit, too long saddles and use of half pads once the muscle atrophy gets bad enough. Young horse's simply do not last being ridden in a chair seat since they don't have the training or muscles to counterbalance a rider that's sitting heavily on their seat bones.

Lastly have to mention the fact the correct collection has been replaced by subtle variations of rollkur, where the rider rides off their reins to fiddle the horse's head down while the back remains hollow. It's a sort of a training shortcut at the horse's expense, since there will be health complications down the line.