Because they hate everything and everyone but expect constant unthinking adoration and obedience from animals. The concept that animals are living, feeling, thinking beings with their own desires and needs just doesn't compute with them, no matter how much they claim it does. They expect animals to give give give and get nothing; they refuse to put in the time to bond with an animal, to train it, to teach it...and certainly not to walk it. So once they realize that animals have needs, too, and aren't just affection machines...they get rid of them, usually to get a new animal because they're sure this time the animal will love them and do everything they want and require nothing. And the cycle continues.