Because they are STUPID and having nothing else to do with their money except line the pockets of the breeders instead of offering a home to a pet that is on death row.
I resent that statement. I got my dog from a breeder, and she is the best dog I've ever had. Yes, she was $700. If it was my choice, I would have gotten a homeless dog. But I think it's the same for breeder's pups. If they don't get bought... where do they go? Shelters, maybe. But all that matters is how much you love your dog, not where you got it from.