The reason why the old man said that dogs cannot be ridden is because dogs are not large riding animals like horses, and their body shape and bone structure are not suitable for carrying the weight of humans. Riding on a dog's back can cause a lot of stress and discomfort to the dog, and may even cause injury or deformation of the dog's bones.

And dogs are lively animals, they like to run around and play. If you ride on the back of a dog, the dog may struggle, resist, or even bite because of discomfort or discomfort, thus posing danger to the rider.