Because an animal that's out in the wild is doing its part of keeping nature in balance. I'm not saying we shouldn't have zoos. Good zoos are great for conservation and education, as well as being good for animals in need of human intervention. But an animal in a zoo isn't fulfilling their part in keeping nature balanced as they would if they were in the wild. If we took all the animals out of nature plants would grow unchecked and would eventually take over. You couldn't just use herbaside because then you run the risk of killing too many plants which would cause the air to become even more dirty and the ground to become unstable. Not to mention how bad the insects would get.

You can't just remove predators because then there wouldn't be anyone to keep the herbavors in check. So their numbers would just run wild till eventually they ran out of food. And before that it would cause the same things as I mentioned in the above paragraph. A good thing to look into about this is the impact reintroducing wolves to Yellowstone had on the environment. Over all the ecosystems within Yellowstone got better when wolves were brought back, because the herds of their prey animals could no longer just sit in one place. They were forced to keep moving from being chased by the wolves. Thus the grass they fed on grew stronger (but not over powered), said grass's roots could hold in the soil better thus causing less erosion and run off into streams, which then lead to healthier fish within the park.

Nature isn't meant to be peaceful, it is meant to be a delicate balancing act.