Because a spayed cat can live 15 or 20 years, and one that has three litters of kittens a year won't?What's worse, throwing out ovaries, or throwing out the dozens of kittens and puppies nobody wants?
My neighbor didn't bother to spay her dog. Of course, it got pregnant, and she didn't want to raise puppies. She took it to the vet, the vet (reluctantly) agreed to spay the dog and abort the puppies. Feel better?