Zoos aren't necessarily a bad thing for animals.
According to a confirmed study, mammals generally have a longer life expectancy in zoos than in the wild. More than 80% of the mammal species analyzed in this study lived longer in captivity than in the wild. And there are many other studies that support this statement. This is because they have access to medical care, a constant food and water source, shelter, and are safe from predators and poaching. Some animals have actually came off the endangered animals list because of this aid from zoos and wildlife sanctuaries.
It is true, zoo animals become bored. They do get depressed because there can be a lack of stimulation for them, some zoos do a fairly decent job keeping them entertained while others don't. There are some private zoos that are just for the cash grab but the goal of most public zoos are wildlife preservation and animal protection. Zoos do have their drawbacks but they also have a lot of positive attributes.