Carnivores tend to sleep longer than omnivores, who tend to sleep longer than herbivores. For a hunting carnivore, energy comes in big bursts, so it makes sense that they would be active for a short period of time, and hoard energy when they didn't need to be active. For a cud-chewing herbivore, time spent not chewing is time spent not creating energy. Obviously, this is a broad generalization - feeding habits, day/night cycles, predator/prey behaviors all factor into a particular animal. But it probably explains why your cat, like the panther at the zoo, spends most of its time asleep.

Also, cats and panthers are crepuscular, active at dawn and dusk. Which leads to lots of waiting for that time of day.