This is legitimately a huge (and still somewhat controversial) topic and I don’t really have the time to go into it in a lot of detail right now but here is a quick summary: 

- Cats are damaging to local ecosystems. Well-fed pet cats still enjoy hunting and end up killing not for food but for fun and to practice their hunting skills. It’s a natural instinct, not malice, but it can absolutely devastate local songbird and wildlife populations.  

- Outdoor cats are at MUCH higher risk of injury from vehicles, other animals, or even ill-intentioned humans (we once rescued a black cat with a broken tail I’m 99% sure had been broken by a group of kids swinging her around by her tail)

- Outdoor cats are at greater risk for contracting communicable diseases like Feline HIV

- Outdoor cats can get lost or even stolen (I’ve had both happen to me before)

- Outdoor cats are at higher risk of suffering from the elements (getting caught in snowstorms, freezing to death, etc.) 

- Outdoor cats are much more likely to contract parasites like fleas, ticks, and worms

- It is much more difficult to spot irregularities in your cat’s health if they are regularly outdoors (you can’t monitor how frequently they urinate or defecate, and whether or not there are any changes to their bathroom habits)

- Outdoor cats are more likely to end up consuming something dangerous (rat poison, spoiled food, etc.)

- Outdoor cats who are not fixed are likely to increase the feral cat population by getting pregnant/impregnating other cats. 

In general, outdoor cats are damaging to local ecosystems and at great risk of premature death. 

Now, that’s not to say you can’t find ways to let your cats enjoy the outdoors, though! Here are some fun solutions: 

Catios: 

Cat harnesses (yes, cats can be taught to walk on leashes!)

Cat-safe plants for nibbles and noms:

Cat “TV” (ie shelves or seats near windows where cats an easily observe the outdoors from the safety of inside)

Basically domestic cats are not safe outside alone, nor is it safe for the local wildlife to have them there in numbers. There is a lot of information out there discussing this you can find on google (and like I said, there are people who disagree) but these are the main reasons I tend to encourage people keeping indoor cats, ESPECIALLY if you are in an urban or residential (non-rural) area where risk of traffic collision is high.