look, it should be obvious that being indoors will lengthen your cat's life, it's not like it's a hard deduction to make.
How about:
An indoor cat gets much less exercise, which can seriously reduce lifespan (especially if they have a constant source of food)
A cat left indoors when the owner isn't home will get neurotic and unhappy
An indoor cat (especially a lone cat) will get bored trapped inside when their owner is not home, and this drives them to play with things that really aren't a good idea (e.g. hanging themselves in curtain cords)
Cats use grass as an aid to help them throw up when they're ill. If they're stuck inside with no access to grass, they can get into trouble (especially if they decide a houseplant is a substitute, which may be poisonous or even plastic)
A cat trapped indoors can only access what food they are given. If they run out, they're forced to choose between starving and eating things that really aren't good for them (many kinds of human food are poisonous to cats) an outdoor cat can get food elsewhere. Cats like to eat/snack very regularly, so being without food for only a couple of hours is enough to piss off a cat.
Ditto
water.
Some cats like to hunt. A cat trapped inside can't hunt, which at best makes them unhappy and at worst makes them hunt live mains cables (which is dangerous
at best)
Now you might say it's about "how much effort you put into your cat" but are
you at home all day? If you work full time, you're likely only home and awake (and not busy cooking/cleaning/etc) for a couple of hours a day. Is that enough? Someone compared a cat to a child earlier (something like "would you let your toddler out on their own"): would you leave your toddler home alone while you were at work? As you can see, they are clearly different. Don't treat your cat like a toddler, treat them
like a cat.
How about you try staying inside your house for 16 straight years. You'd go crazy.
Cats aren't stupid. For the most part they can look after themselves outside. They are, after all,
animals.
That's not to say there are places where an indoor-only cat makes sense. For example in the US, where people shoot things for fun, it might not be safe to go outside at all.
(or more seriously, in cities or near major roads)
I may be biased from the fact that we've have always had indoor/outdoor cats, but that's normal practice in the UK. They tend to make themselves voluntarily indoor-only cats when they get old, and then they die of old age. They don't get shot, or hit by cars, or taken away by animal control (even if a cat of ours did get picked up, it would have its microchip scanned and returned to us immediately, though it's never happened). They don't get tortured (this is a criminal offence). They don't get given drinking water laced with LSD.
If I lived in your neighbourhood, damn right I'd keep my cat inside. But you can't just extrapolate that to the entire world.