The Phoenix Zoo is the largest privately owned, non-profit zoo in the United States. Here are activities that they have previously engaged in and are currently engaging in:
Since opening in 1962, the zoo has focused on animal conservation efforts. Programs at the Phoenix Zoo include captive breeding for reintroduction (for instance the Arabian oryx, black-footed ferret, Mexican wolf, Ramsey Canyon leopard frog, and thick-billed parrot), sanctuary for animals that are no longer breeding (for instance the African wild dogs at the zoo), and rescue and rehabilitation (for instance lead poisoning treatments for the California condor).
The Phoenix Zoo participates in the Species Survival Plan (SSP) for 29 species with the Association of Zoos and Aquariums. The goal of the SSP is to engage in animal husbandry and research projects for selected species that are in need of conservation efforts.
The information provided above clearly demonstrates that not all zoos do more harm than good. Your generalization as the OP on this question / statement wasn’t very good, unfortunately.