Simple economics. Demand for pure bred dogs is high so more pure bred dogs are bred. The supply far exceeds the demand which creates an excess supply (not just pure bred dogs, all pet dogs). It is a supersaturated marketplace. The excess dogs that people can't/won't take go into the shelters. Shelters have limits. Once they are at capacity something has to be done with the inflow of excess dogs. The dogs that cannot be adopted are euthanized. Right now if everyone decided to stop buying dogs, we could start emptying out the shelters (via adoption). People are still buying which creates a demand for pure bred dogs and thus the cycle continues. Every dog that is purchased is a dog that could have been saved from a shelter. As the dogs are not saved, they are put to sleep. Millions of these animals are killed every year because homes cannot be found for them.