For LOTS of reasons.
In my city, it is not legal for your cat to roam off your property. They kill native wildlife, including songbirds and small mammals, spread disease to both wildlife and other pets, contaminate water sources, crap and dig in people's gardens, upset other peoples' appropriately contained cats and dogs, get into trash, and generally pose a nuisance. So far none have been stupid enough to get into our yard. If they did, and managed to survive our dogs, they'd be trapped and dropped off at animal control so their owner could pay a hefty fine. If you want to allow your cats outside, build a catio or install cat proof fencing to keep them in your own yard.