Many if not most zoo animals are not capable of returning to the wild. Either they were born in captivity, or they have an injury or disease that would mean they would not survive their natural environment. It can be sad to see an animal in captivity, but releasing an unfit animal into the wild is probably more cruel than allowing them to survive in captivity.

That said, not all zoos provide adequate environment, diet, healthcare, and enrichment for their animals, so it's important to do your research and learn what animals need to thrive, so you can make an educated judgement.

On the other hand, zoos are an excellent tool for keeping critically endangered species alive and reintroducing them to the environment! I recently visited the Santa Barbara zoo, where they had a large California Condor exhibit. The condors there were flightless due to injuries or deformations, but they and some other captive condors at other locations were able to produce offspring- that were then released into the wild, helping to bolster their near-extinct population.