Because it takes a lot of close breeding to maintain a certain phenotype. More so when the phenotype is further from what works in nature such as in the case of big large boned dogs. Also the psychological makeup or what we call temperament of the domestic dog is contrary to what works in nature. The qualities we look for in most dog breeds fit for one job or another just by being present indicate the dog is missing other qualities that would allow it to survive in the wild. Wild canids avoid conflict in all but very specific circumstances, especially with humans. It is not in the best interest of a wild animal to enter into conflict unnecessarily. Even if it wins it may be injured to the point of not being able to hunt for food and will die. LGDs obviously will enter into conflict willingly, this is not arrived at through natural selection, it is done by the hand of man.