The animals are trapped in the zoo from the wild. Some of the zoo don't have such a good service, So the animals can gt sick. I'm sure that animals won't like being trapped and watched by someone- it is better to just let them be in the mother nature.
Animals in zoos are treated very well and given everything they need, Granted they lose the ability to hunt but they get food, Water, Shelter, Health care and attention. Also most zoos now have animals in zoos to help with wildlife conservation so putting animals in zoos helps prevent animals from going extinct
When animals that could very easily be going extinct due to human action such as poaching, It is far better to take them in and to look after, Raise, And release more of them back into the wild than do nothing. Also, Zoos educate the common people about endangered species, And by teaching them things like these, They can easily help donate to animal charities and suchlike.