Because, along with birds, cats kill a lot of rodents. Rodents both carry disease and eat stored human food. Thus humans have, historically, been better off with cats around, and may in fact still be better off.
If a rat comes in my garden I can kill it. If a cat comes in my garden regularly, sprays on everything, shits where my kids play ... then legally (UK) it seems there's very little I can do; I don't see why.
Cats are trash for rodent control. They spend too much time playing with their 'food' and not enough time actually killing it. Dogs are much better at killing rats yet we don't let dogs roam around outside willy nilly.
Most rodent control in the modern era comes from modern building techniques and materials, as well as waste disposal networks, that keep rodents out of our buildings and away from human sources of food. Go to a neighborhood with well maintained buildings but no cats and you'll find fuck-all rodents. Go to a neighborhood with run down buildings and a million cats, and you'll find a billion rodents.
And cats leave gory corpses of small animals in front of doorways. At least, they do whenever they're actually doing their supposed job of killing rodents. In practice it's enough corpses to make a mess but not enough corpses to actually hamper the rodent population.
The excuses made for cats seem to nearly always be characterized by a lack of critical thought. Saying that is probably considered controversial, but so is the toxoplasmosis hypothesis.
I was trying to answer "Dogs are much better at killing rats yet we don't let dogs roam around outside willy nilly.", but I was not as precise as I could be in retrospect.
The objective function that most human beings use is not "what animal kills rodents most efficiently", but "what animal kills rodents most efficiently and poses little harm to human beings most of the time". In my experience, most stray cats will likely run away from children and most stray cats will likely not initiate attacks against children. On the other hand, most stray dogs are more likely to do both of those things. As a result, cats get a pass for being less efficient rodent killers than dogs because they are also less likely to harm human beings.
> The objective function that most human beings use is not "what animal kills rodents most efficiently", but "what animal kills rodents most efficiently and poses little harm to human beings most of the time".
See, I disagree. I do not believe that killing rodents is a serious consideration for most modern cat owners. There are cheaper, more efficient and lower maintenance methods of exterminating rodents than cats. Most cat owners own their cat as a pet and think nothing of letting it terrorize the neighborhood birds because it's cute. Talk of rodent extermination is a post-hoc justification for their behavior.
Oh, I agree with that: most people who have cats have them for companionship, and they don't think about the things that said choice externalizes. However, the same can be said for people who have dogs for companionship. Some dogs routinely lunge at people passing by. In addition, despite the best efforts of those who walk their dogs, there are smears of dog feces on sidewalks. Both situations are examples how keeping a dog has externalities.