With all due respect, do you think a clownfish stays within six, eight or even ten feet of his anemone in the ocean? He’d starve to death.
They do stay pretty close. That, and there's something that explains why plenty of animals can be perfectly happy in (good) zoos: the distance an animal travels in the wild is not necessarily a distance it needs to travel for its exercise and enrichment needs. More often it's for food, water, and mates.
Think of it this way: if the only place you could buy food was a 5-mile walk away, and you didn't have a car or food delivery, you'd walk 5 miles to get food. But if someone put a good grocery store a block from you, you'd go there instead. If I put a food dispenser right next to a wild clownfish's anemone, and prevented anything from turning up that it would have to chase away, I guarantee it'd stay right by the anemone.
Now, animals do have a certain amount of activity they need. But one of the tricks to keeping anything happy in captivity is to figure out how much activity that is.
(And on a different and somewhat irrelevant topic, indoor cats live way longer on average, and are protected from just about every predator and disease out there. Cats allowed to free-roam outdoors are incredibly destructive to the environment, as well, killing millions of birds every year. Cats can be perfectly happy kept indoors, with outdoor access from a catio, harness training, or just supervised outside time if they don't bolt. And yes, you can train cats to walk on leashes. You just can't slap the harness on them and expect it to work. Imagine if you had a dog that had always been in a fenced backyard, never on a leash, and you tried to walk it on a leash. Wouldn't go well. Same idea applies to cats on leashes.)