They also need more regular play since an indoor environment doesn't present the ever-changing environment found in a garden. British behaviourists report more problems in indoor cats than in outdoor cats. This does not apply to sexual behaviour since neutering removes the reproductive urge rather than thwarting the desire. I leave the door open and they are usually back in the house in a shot. And they rarely show any interest in going back out.
Some owners take their cats on supervised outings which the cats enjoy.
We take our cats out on leashes into the city fairly regularly. We have only once seen anybody else with another cat on a leash in all the years we have been doing this. People are missing a wonderful experience. Now they have the idea of a carrier as a means of going out to fun experiences. There are those who argue that any access to outdoors, whether on a leash or in an enclosure, is irresponsible as it exposes cats to risk of infection.