Because there are millions of wonderful dogs in shelters all over the country that need good homes, or else they may be euthanized. There are plenty of purebred dogs and puppies in shelters, and of course there are also lots of mixed-breed dogs.
You should know that dogs in shelters might not present their true personalities while in the shelter. Rivers was rather timid during her first few weeks with us. As she became more comfortable and confident in her new home, her true personality emerged: outgoing and perhaps a bit pushy/persistant (in the cutest way!). So expect an adjustment period after taking home a shelter dog.
Once you've found a dog you think would fit into your family, you should ask shelter attendants about the dog's history, if known. Maybe the dog has been adopted already and returned, and you'd want to know why. Maybe the dog came from a loving family whose living situation changed, so the dog would be used to home life. Or maybe the dog was a stray, or its history is unknown. The more you can learn about a potential new family member, the better.