The short answer to this question is “because you can!” Leash corrections and other tactics that are part of correction-based or traditional training methods “work” because they are at the very least uncomfortable for your dog. Progressive training and training based in positive reinforcement can get the same results with less stress for your dog, and avoids some of the potential hidden consequences of more old-school techniques. Some of those consequences can include increased or redirected aggression (e.g. a dog may growl or snap at another family member or person passing by), increased generalized anxiety, and behavioral “shut down” where the dog reduces all of its activity in response to stress. These side effects are virtually non-existent with the techniques we teach in classes and private training.
Positive reinforcement training is becoming increasingly popular. As it becomes more accessible, more top show dogs (agility, conformation, obedience, flyball, etc.) and working dogs (bomb sniffing dogs, assistance dogs, etc.) have been trained using positive methods. They really do work!! Using positive methods is also a lot more fun for both parties, and builds a stronger relationship with your dog.