Because pets and stray pets are an ecological disaster, especially if they freely reproduce in the wild. It’s fine if you keep them at home, but not if you let them roam free. cats will kill anything they come across just for fun. entire species and ecosystems have been wiped out because of this.
Our neighbor had freely reproducing barn cats that he didn’t care about at all, so you’d have sickly cats all over the neighborhood, regularly finding dead ones. It isn’t fun for them either. It’s best for everyone involved to leave their reproduction to breeders.