well there's no medical reason they need to go out. but i've met more cats who wanted to spend at least some time outside than cats who never did. though, yes, i've met cats of both kinds. cats of both slants live with me now, even!
keeping them in protects them from a lot of potential harm - from traffic, from wild animals, from other cats, from people too daft to keep their dogs contained. etc. so probability-wise on average they live longer. but i have a 24 year old cat here who spent most of her first 10 years outside, and gradually decided she didn't want to bother with it much anymore. and i have a cat around 15 who comes in and out as he pleases, and always has. now, in the summer, he spends a lot more of his day inside in the AC, and his nights outside, when it's cooler, and there are fun things like toads to chase. in the winter, when it's cold, he spends more time inside overall, where it's warm. but overall he's outside probably 75% of his time.
it's true that his chances are worse being able to go in and out as he does, but he's an old cat whose gained a lot of kitty-wisdom over the years and knows how to generally look after himself - we're in a very low-traffic neighborhood and have a safe fenced yard ((tho he jumps it, its a safety-zone from certain other critters)) - which certainly helps. there are a lot of factors in his favor, but i know he's been blessed to have gotten to where he is, and been safe all these years.
letting cats in & out used to be a no-brainer for me, but heartache has caused me in more recent years to keep our newer little ones in, unless i go out with them, and then just those i trust to come when i call. i hate to deny them all the joy they get from the outdoors, but i hate to fail to protect them, at the same time, you know?
& then i have a couple old tomcats who have spent all their lives outside, whose routines i'm not going to challenge.
