It is a partly a matter of appearances, with zoo animals less active at peak visiting hours, and partly a matter of normal rhythms of carnivore life. It is said that lions in the wild are normally inactive for 20-22 hours a day because they need to conserve their energy for hunting. They can never be sure where their next meal will come from.
As for the house cat, although it does know when the next can will be opened, the basic behavioural pattern is the same. Even a cat that has never seen real prey will stalk a butterfly through the window.
Young animals can afford to do that, but from an evolutionary point of view, old lions especially must conserve their energy for the business of surviving. At some point of life, their metabolism does begin to slow down, and a sedentary cat may become overweight.
In general, many zoo animals are from hot climates and are most active in early morning and late afternoon. They sensibly lie low from 11-3 on a hot day, so visitors should try to come early in the morning. Zoo animals have the benefit of a nutritional staff that prepares diets as close to natural as possible. Modern zookeepers also make sure that animals mimic natural behaviour, living in groups, with plenty of space, and foraging and competing for food. Some do get chunky, like the dominant animal in a group that always gets its fill, but most stay lean.