They would be expensive to raise by yourself as a food animal, but many people are more than happy to feed outdoor cats all on their own. Plus they do scavenging in the trash and are very efficient hunters of local animals like songbirds and frogs. So though it would take money to raise and fatten them yourself, it’s cheap as free to just yoink them off the street if you don’t get caught since others have done that work for you. Many people are accustomed to letting their cats go around unsupervised outside (even though outdoor cats have a third of the lifespan on average as an indoor cat) so that’s more opportunity to nab them without being contested.