[alt text: an illustration of a person with a head-empty expression on their face, who is saying, "Not letting your cat outside is CRUEL!" Around the person are various gruesome scenes of different cats in distress. From the top and going clockwise, the scenes include: a cat being carried away by a hawk; a cat that is on fire; a dead cat in the road that has been run over by a car; several dead kittens; a cat that is missing an eye and various patches of fur; a cat that is feasting on a songbird; and a cat that is being carried away by a coyote. The person appears to be completely oblivious to these scenes of distress.]
It is. They need to be outside as much as you do.
Nobody thrives in an exclusively indoor environment. They go crazy. Sterilization can help a bit, but they still need to go out sniffing stuff and at least play at hunting daily.
There's a reason even prisons have courtyards and outside activities. It's downright dangerous for everyone if they didn't.
I also rarely go outside, I am a perfectly normal human.
And that's a choice.
But animals tend to enjoy the outside. They don't surf the web, watch TV/movies, play or listen to music. They need stimuli, outside input.
Edit: I do go outside and not even I claim to be perfectly normal. XD
Maybe yours do. If that is the case, take them for a walk on a leash or in a covered stroller. Build a covered patio. You could even play with them in your yard and bring them inside once you can no longer supervise them. There are many, many, many ways to provide this enrichment without leaving them to get hit by a car or scooped up by a predator.
Skill issue. My cats haven't been outside since they were adopted, and they live happy, fulfilling lives full of enrichment. That's what cat toys and puzzle feeders are for.