Zoos do not keep animal purely for our curiosity. Most zoos have breeding programs for threatened and endangered animals. Also, although the research could be seen as fulfilling our selfish curiosity, what we learn is used to help animals and better understand our world.
I do think that animals are kept extremely well in most AZA zoos. It could always be better. Yes, they are confined, but they don't usually have to worry about predators, they don't have to waste energy on hunting/foraging (or it is significantly decreased), they are less subject to environmental issues, such as drought, famine. extreme temperatures and they get the top veterinary care. Most animals live longer in captivity than in the wild.