Cats aren't stupid animals. My family had a cat for 16 years, adopted him when he was about 2 years old. He lived a long, happy and healthy life being able to come and go as he pleased. On more than one occasion we'd see him hop over the fence to the neighbors house and see the neighbours feeding him. There were other cats that roamed our neighbourhood and they'd occasionally fight. My cat would also hunt in fields nearby and bring back his prized catches.

To contrast, I have a friend who has a cat that doesn't dare step beyond the bounds of her back garden. Also a perfectly happy and content cat, just not that interested in going outside.

They're not all the same. Saying that all cats should stay indoors is a terrible generalisation to make and could result in a very unhappy animal.