There are a number of reasons why dogs are not as flexible as cats. One reason is that the skeletal structure of a dog is different than that of a cat. A dog's bones are denser and heavier, which makes them less able to twist and turn their bodies the way cats can. Additionally, dogs have shorter limbs in proportion to their body size compared to cats, which also limits their range of motion. Finally, muscles play a role in flexibility, and dogs simply don't have the same level of muscle development in their hindquarters as cats do. This lack of flexibility means that dogs can't perform some of the same feats of agility that cats can, such as leaping great distances or twisting their bodies mid-air.