Wouldn't it be nice to be able to sleep as much as a cat does? Oh, the life!
While most of our cats do live in a cushy environment, this behavior is actually built in. If your cat could talk: "Why waste energy and calories by being awake and active if it's simply unnecessary?"
It used to be that cats had to conserve their awake time for hunting, mating, and socialization. This goes back to when they were predators -- hunting, eating, and sleeping. Cats often sleep much more than their prey does. By contrast, prey animals usually need to constantly graze to obtain their calories and also have to be on alert for predators. They generally only take "catnaps" for a few hours at a time.