Same reason there are tens of thousands of unwanted pit bulls. They get an "image" and then find out the actual dog is not so easy to live with, needs to be trained & needs an owner who is experienced with dominant breeds. (How many times do you see that here where people are told a GSD, pit, SIbe, Dobe, Chow... is not a good choice for an inexperienced owner- well a lot of them get one anyhow.) They do not realize how much work raising and training a dog is nor how expensive. Most of us have bought something we realized later was a dumb idea, not what we thought and so on, it's just that most of us do not take getting a pet lightly. Some people do.
Dogs are expensive and a lot of work and some people do not realize this and puppy mills and BYBs are more than glad to sell anyone who has the money a dog. They do not train that dog and it tears up the house and is obnoxious, so it gets tied up outside because they can not put it on the curb for the trash pickup, but they might as well.