Because all the proposed solutions are doomed to failure really. To actually begin to control the feral cat population (which you do need to do, just because they make good pets doesn't mean they're good to have out in the ecosystem) you first have to stop humans from introducing more on a constant basis. As long as any dipshit can buy their precious little kid a kitten for christmas then dump the cat into the wild once it's grown up and they don't want to care for it anymore, you're going to have a feral cat problem.