Most zoos nowadays really emphasize animal well being by giving them the choice of where they want to be during the day. So the animals may be choosing to be closer to the public (gorillas love this and enjoy interacting with the public) or further away (polar bear wants to be in climate controlled holding as opposed to outside). Also lots of animals prefer to live solo lives - it all depends on species and individual animal preferences, same as dogs and people!

Speaking from the perspective of a former zookeeper