Most of them aren't currently doing all that bad. But some of them are and most of them eventually will be miserable in the future. Point at a random lion or wolf in the wild and they are happy, loaded with itchy parasites, but fine. Eventually they will all either get sick or be injured. Prey animals die quickly, ripped apart by the lions and wolves. Top predators are doomed to suffer. Their final days, weeks, sometimes years of suffering with a debilitating injury are not pleasant. That misery is what a good zoo removes as compensation for captivity. It is the least we can do. Whether that exchange is fair is a judgement call, but when making it we must remember that our ideals of nature and natural existence are not necessarily universal. Many a wolf might gladly live inside a fence in exchange for never being hungry, cold or itchy again.